On one end of the scale we've got "gore", which creates a sense of disgust and horror based on mutilating the human body (or other bodies) and appealing to the audience's own sense of fear and dread at having something similar happen to them. On the other end, there's "gorn", or "gore porn", which is gore for the sake of gore. It can be hard to judge the difference between them, because it's largely based on intent rather than severity or style, but the former is meant to have some value to the story, and the latter is meant to be "appealing" in its own right.
One important aspect of gore is that it's an immediate, reliable, visceral reaction. Seeing gore can make the skin crawl in ways that merely hearing about "death" cannot. If you heard that a soldier died in a battle, you might not think much of it. If you hear in excruciating detail about how the bullet broke through his skull and punctured his brain, you may feel differently. The basis of concepts like finger damage and eye damage - things that may disgust you even if you just hear about them - are founded in intrinsic connections to identifiable parts of your body. You can almost always feel your fingers, and they're one of the most sensitive parts of your body. Therefore, if you hear about, or see, damage to a finger, that generates an immediate response. That's what gore can do: fill in for the sense of touch by immediately connecting to your real sense of touch.
This also creates a divide when talking about things like cartoons or anime, where the human body is strange and stylized. It's almost unbelievably unpleasant to watch a needle near a real person's eye - it's much less unpleasant to see one near an anime character's eye, because it doesn't look like an eye. In the same way, a lot of "stylized gore" ends up not connecting with the audience, because it doesn't look like damage that real people would take. Look at this clip, for example (gore warning, if it wasn't obvious): there's so much blood, and so little bone/muscle, that it makes me laugh. They don't seem like people, they seem like pinatas made of meat. It's like watching a video game model get torn apart. In contrast, this scene (also from a gory anime, naturally) relies on a realistically detailed finger - without that detail, the scene wouldn't be nearly as effective.
Anatomy is an important concept when it comes to averting the Nerf effect. Having realistic anatomy puts weight and damage behind an attack, because it relies on understandable, relatable concepts. When arms are sliced off in an anime scene or, heck, in the Star Wars movies, it's a clean cut. There may be blood, sure, but it still basically seems like a tentacle chunk in the shape of an arm. There's no sense of bones or muscles, just a big floppy "arm". In fact, most fantastic gore is like that - big bloody chunks, like something out of a Looney Tunes short. There's no sense of the constituent parts, or of any underlying skeleton: you just slice parts off like you were cutting up a roast. When you touch your arm in real life, you can easily feel the bones at its core. Media tends to depict it like it's only flesh, which makes it feel "unrealistic", at least in the sense that you can't relate to it. In contrast, a well-depicted arm break (complete with the sickening crack of bone) is generally far more effective. Compare this real picture (gore warning) from WW2 to the usual depiction of de-limbing: even though the end result is the same, the fact that the arm is a real arm connects it to all the properties of a real arm that are not present in a cartoon or anime sequence.
One particular type of damage that I feel is effective is damage from a cannonball. A cannonball is heavy in a way that the brain can relate to, and when fired out of a cannon that heavy thing is moving incredibly fast and bashing whatever gets in the way. On the other hand, it's also slow enough that you can see it coming - which, upon reflection, is terrifying. It's not like a bullet, where it's small enough that the brain might not even register it (when watching it, that is - presumably the person being hit is distinctly aware of it). A cannonball is large enough that if the brain has any idea of the weight of the thing, the idea of a limb being smashed off by one is as visceral as it gets. And it's not a clean cut either: bones are going to shatter and break as the cannonball impacts the leg. It's not just unpleasant (since I would say that most forms of amputation are unpleasant), it's unpleasant in a way that the brain can easily relate to.
In essence, damage and gore are the natural result of an object hitting the human body. Therefore, both the object, its speed, and the body are in play. The speed might be the most difficult part to understand in an abstract sense, but it's easy to hold a knife or a baseball bat, and it's easy to think about your body and how it would react to those sorts of stimuli. In fact, it's so easy that it's basically the whole point - your body feels uncomfortable even without you directly needing to think "that would be uncomfortable". Translating something unimaginable like "being slashed by a sword" into something brutally comprehensible like "getting your finger whacked off by a butcher's cleaver" is a major key to evoking emotional responses with fictional situations.
As mentioned above, war movies tend to get a pass when it comes to gore censorship:
"In both films [Schindler's List and Saving Private Ryan], the content is not meant to shock, nor is it gratuitous. We applaud ABC for letting viewers know ahead of time about the graphic nature of the film and that the film would be uncut." - PTC president L. Brent Bozell
This is a major issue when it comes to "mature themes": is the theme in question being used for cheap shock value, or to really examine concepts and real-life events? Saving Private Ryan is an acceptable movie because stuff like that really happened (and happens), and depicting it helps people to understand the horrors of war. Is there some substance to, or reason for, the gore other than "blood is cool"? This, I think, is one of the defining lines between gore and gorn. Gore can provoke an emotional and sensory reaction, yes - but what is the purpose of that reaction? Is it an escapist form of titillation? Is it to revel in the reviled and forbidden? Or is it to try to empathize and sympathize with the victims of violent events - to understand their plight?
The context of the gore is the deciding factor, and this is why those two movies were acceptable. Schindler's List and Saving Private Ryan (whatever problems I may have with the latter) are about victims. The scene on Omaha Beach is not meant to be "totally badass action sequence", it's meant to be young men (American and German) being cut down in the prime of life. Making up an atrocity to show scenes of rape and slaughter would be excessive if it wasn't justified, but showing the horrors of the Holocaust is acceptable because it has real historical implications. The goal of the concept is to make sympathy, rather than to allow people to revel in, and celebrate, atrocities and violence. Of course, the concept of audience interpretation means that such things cannot be avoided, but the intended tone of the product does play a major factor.
The thing about gore in general is that it's unavoidable if you're being realistic. Yes, it's certainly possible to over-exaggerate, but people are full of bones and guts and blood, and pretending that we're not going to react negatively when being hit by a sword or a bullet is itself unrealistic. If the assumption is that people in a setting have normal human bodies, then their bodies behaving in "un-human" ways is going to be weird. This ties into the concept of "hard but fair": when bad things happen with a logical background, it's more justifiable and sympathetic than when bad things happen for absolutely no reason. If there's fountains of blood, it's going to seem intentionally over-the-top. If there's a reasonable amount of blood for the injury type, it's going to feel more like the natural result of damage to the human body.
Realistic damage and the negative reactions it provokes can be a major humanizing element for a given character. It's easy to not care when a group of stormtroopers is slashed up by a lightsaber, but only because it lacks emotional connection - there's no sense of pain, fear, or terror, and there's no damage that can be connected to plausibly. It's faceless, armored soldiers being slashed up by special effects and falling down; there's nothing for the audience to relate to. The aspect of automatic emotional responses works better when there's other emotions and elements to work with, and those emotions will feel more real if there's a layer of logic and plausibility underneath them.
Women and Gore
"Female soldier" isn't an uncommon character profession in most modern works. The age of "women in the kitchen" has largely passed by, and in most cases it's expected that a female character with combat training will be at the forefront of the fight along with all the more traditional male characters. Averting this concept would seem backwards, and can result in accusations of sexism. Yet, as poorly represented as soldiers generally are, "female soldiers" for some reason get it even worse in terms of characterization. They rarely feel like "women" and "soldiers" simultaneously - the latter has to be emphasized for the sake of some feminist ideal, when in actuality "being a soldier" should be more alienating to the audience than "being a woman".
An important point about this is that women are rarely treated "equally" to men when it comes to gore and violence. There are plenty of movies and books about a lone woman striving to prove that she's equal to men, but comparatively less about female soldiers fighting and dying alongside the men in equally grim and unpleasant circumstances. There are a few games and movies with female soldiers being killed alongside men, but they're comparatively rare. Of course, it wouldn't be uplifting to read a story about a woman going into combat and immediately dying, but essentially it's an unfair concept (in addition to the unfairness of the protagonist shield concept). How can they be judged by the same standards if one group is able to die and the other isn't?
There's a negative societal reaction to female gore in general, since "women" still generally fall under the automatic sympathetic heading, along with "children". The idea of female extras being killed en masse is considered upsetting. Hence, even settings where women are considered part of the normal army will shy away from showing the same graphic deaths for women as they would for men. A work that does depict a blood or gory death for a female character is usually suspected to have some underlying misogynist or fetishistic motive - and many of them do. This creates an imbalance where it's perfectly okay for women to be soldiers, but it's not okay for them to die like one.
When I talked about Dragon Quest, I noted that the difference between the male and female warriors suggested that a man is expected to take damage, but a woman is not. If the woman was expected to be cut or injured in the same way as the man, it would be insane not to wear the same amount of armor as him. Instead, women are often limited to superficial damage. It's hard to determine whether this is misogynist or misandrist, but it's definitely unequal. If men and women are meant to be treated as equals in combat service, they should be treated as equals in the unpleasant aspects of that service as well.
For example, I liked the character Emma Honeywell in The Last Remnant, because she was a sensible, down-to-earth character for whom "being a knight" was more important than "being female". This was reflected in one scene (major spoilers) where the way she fights is identical to what a male character would do in that position - no damsel-in-distress syndrome, no upper-arm-grabbing, nothing. But then I thought about it, and she's still one of the few female knights in the game. The generic soldiers, at least in this scene, are entirely male. She's a hardened knight with years of experience and a no-nonsense attitude, but she's also a protagonist, and thus gets a dignified protagonist's death instead of being crushed into a bloody pulp off-screen. There is at least a sense of her tiring and being wounded, but it's still something to think about.
On the other hand, there's "Aliens", a movie centering around a mixed-sex group of space marines. There were focal male and female characters and disposable male and female characters. The film did a good job of making it all seem very natural. All of these characters are marines, and they're all at risk, but some of them are male and some are female. Their gender is barely even relevant to their role. I've quoted James Cameron in the past stating that the consistency and tone of Aliens is what helps it succeed, and I think that includes this element of it. There's not a lot of meta-thinking or meta-justification - no "well x character didn't die because she's a girl". It all feels very real and very logical to the characters, and that reflects on the audience.
What's especially interesting about Aliens is that Vasquez was written as a male character, but the gender was changed for a new dynamic. That's the approach I would say should be taken to the whole affair - don't write male marines or female marines, write marines. Their profession, environment, and skills should influence them more than their gender. If their job involves hazards and death, that should be represented, not just to be "fair" but to accurately represent the difficulty and strain of that profession. Above all, things should be equal.
To Sum Up:
1) Gore is a visceral concept that ties directly into immediate reactions of disgust and pain, which is an effective tool in manipulating the audience.
2) Gore can be effective in displaying how truly awful a situation is, and thus create sympathy for the characters caught up in it - but it has to be logical and reasonable, not over-the-top.
3) An unfair situation regarding gore (i.e. female characters don't die in the same way) creates an imbalance that undermines the logic of the situation, making the gory male deaths seem fake in comparison. Therefore, as unpleasant as it may be, consistency is a key element of making gore "believable" rather than "extraneous".