You can't necessarily argue that the inability to attack while moving is even an integral part of the horror mood. The Silent Hill series has allowed you to attack while moving ever since its inception (unless you're using a rifle or extra-large melee weapon) and it generally doesn't make much difference, you're still crapping yourself in combat. On the other hand, the inability to move and fire in Resident Evil seems more a technical limitation since you need to be able to aim manually (up and down in the original games, precise targetting in RE4). You need to be careful about complaining about technical limitations, otherwise you might as well ask why Silent Hill has so many broken locked doors in it (which would, if you asked them, cause the programmers to scream at you "how many bloody empty rooms do you want us to code, huh?!")
Camera is an interesting issue. How many people felt a chill run down their spine the first time they started playing Silent Hill and the camera cut to a hi-angle shot which swung around Harry as he ran around that back-alley corner towards the dead end where the monsters drag him down in the opening? Purely a cinematic shot, since it's just an empty alley, but it's genuinely disturbing- it throws you off balance and gives you the horrible feeling you're being watched. This effect was notably replicated earlier in Alone in the Dark: The New Nightmare (no, not that horrible modern screw-up, the one from LAST generation) in a shot where while walking around the outside of the mansion the camera cuts up to a corridor INSIDE the mansion looking down on your position through a window. If that doesn't unerve you, nothing will. On the other hand, we have games like Doom 3. Being an FPS, Doom 3 obviously has no camera, which means you can only see what's directly in front of you- very inconvenient when a Trite or a Maggot has crawled down the wall behind you. After 4 years after its release I still play it occasionally and I'm still constantly swinging my gun around checking and rechecking every corner as I creep along, no matter how big my arsenal gets. Technically, Resident Evil 4 attempted to recreate this effect with the "over the shoulder" camera angle, only deciding to keep the protangonist in the shot at the same time (since Leon is a character in his own right, not just a faceless avatar for you to project yourself onto in the same way as the Doom marine).
Of course, as has been suggested, the key to horror is helplessness. I neve really felt scared during Parasite Eve 2 because the ability to immolate my enemies with devastating bursts of Parasite energy (and the fact that the basic types of ammo were infinite) never made me feel helpless enough- at least, not until the endgame when the most powerful enemies in the game suddenly spawned everywhere and even then I was more nervous than actually scared. On the other hand, I was rarely that scared in Resident Evil either, although this was because I could evade most of the zombies if ammo became scarce- although being forced to count every bullet certainly added to the tension, you handled it in a clinical fashion. One of the examples of what IS scary in Resident Evil? Nemesis- being chased from room to room by a recurring boss who actually attacks you outside of boss fights (yes I know his fights are all scripted, but still...) You can drop him if you have enough ammo, but he'll still be back and he's a deadly opponent every time you meet him, making you feel genuinely threatened by him. On the other hand, you have to be careful when attempting to make the player "feel helpless" that you don't do so artificially- the most egrarious example I can think of is the Fatal Frame series. I'm not saying it's not scary (it's terrifying) but attempts to promote the horror element by emphasising that "rather than a gun-toting soldier, you're just a little girl with a camera" miss the point that the ghosts aren't trying to wrestle you to the ground and eat your entrails, and that a grenade launcher wouldn't have any effect on them anyway so a camera is all you need. From the ghosts' perspective, you ARE packin'!
Finally, let me mention one game which I think IS truly terrifying, and is regularly overlooked on scary game lists (probably because of its frustrating design choices)- Siren/Forbidden Siren. I never actually played too much of this game, largely because it scared the living piss out of me to the point where I just couldn't go on (also because the puzzles were so damnnably obscure I felt I'd never beat it unless I practically played it with FAQ in hand, which kind of ruins the mood). The most nightmarish part is the enemies- the shibito. Basically Japanese zombies, the twist is that they cannot die. A lot of the time your character doesn't even have a weapon (since you control several different characters over the course of the story) but even when you do the shibito can only be stunned at best (although there is an exception when you shoot one causing it to topple into a well, although that's actually a highly obscure puzzle solution). Even worse, if you have a gun firing it DOES actually attract other shibito in the area, which can potentially make matters worse. Fighting is therefore not the answer (one of the things that actually turned me off the game was a mission where I had to be forced into combat with a shibito to proceed), but stealth (your usual recourse) is made difficult by the camera (which actually does a reasonable job of restricting you to what your character would be able to see, although a lot of it is technical problems). This is solved by the game's main gimmick- sightjacking, allowing you to locate the shibito by tuning into their vision to see where they're looking. Putting yourself behind the groaning, panting, bloodthirsty gaze of a zombie is a disturbing experience which can really freak you out, especially when you're jumping around between several of them, trying to work out the timing for you to creep from one piece of cover to the rest. Finally, the aforementioned fact that the game allows you to control several characters at once should make one thing self-evident- DON'T get too attached to them, the game can be very cruel...
Camera is an interesting issue. How many people felt a chill run down their spine the first time they started playing Silent Hill and the camera cut to a hi-angle shot which swung around Harry as he ran around that back-alley corner towards the dead end where the monsters drag him down in the opening? Purely a cinematic shot, since it's just an empty alley, but it's genuinely disturbing- it throws you off balance and gives you the horrible feeling you're being watched. This effect was notably replicated earlier in Alone in the Dark: The New Nightmare (no, not that horrible modern screw-up, the one from LAST generation) in a shot where while walking around the outside of the mansion the camera cuts up to a corridor INSIDE the mansion looking down on your position through a window. If that doesn't unerve you, nothing will. On the other hand, we have games like Doom 3. Being an FPS, Doom 3 obviously has no camera, which means you can only see what's directly in front of you- very inconvenient when a Trite or a Maggot has crawled down the wall behind you. After 4 years after its release I still play it occasionally and I'm still constantly swinging my gun around checking and rechecking every corner as I creep along, no matter how big my arsenal gets. Technically, Resident Evil 4 attempted to recreate this effect with the "over the shoulder" camera angle, only deciding to keep the protangonist in the shot at the same time (since Leon is a character in his own right, not just a faceless avatar for you to project yourself onto in the same way as the Doom marine).
Of course, as has been suggested, the key to horror is helplessness. I neve really felt scared during Parasite Eve 2 because the ability to immolate my enemies with devastating bursts of Parasite energy (and the fact that the basic types of ammo were infinite) never made me feel helpless enough- at least, not until the endgame when the most powerful enemies in the game suddenly spawned everywhere and even then I was more nervous than actually scared. On the other hand, I was rarely that scared in Resident Evil either, although this was because I could evade most of the zombies if ammo became scarce- although being forced to count every bullet certainly added to the tension, you handled it in a clinical fashion. One of the examples of what IS scary in Resident Evil? Nemesis- being chased from room to room by a recurring boss who actually attacks you outside of boss fights (yes I know his fights are all scripted, but still...) You can drop him if you have enough ammo, but he'll still be back and he's a deadly opponent every time you meet him, making you feel genuinely threatened by him. On the other hand, you have to be careful when attempting to make the player "feel helpless" that you don't do so artificially- the most egrarious example I can think of is the Fatal Frame series. I'm not saying it's not scary (it's terrifying) but attempts to promote the horror element by emphasising that "rather than a gun-toting soldier, you're just a little girl with a camera" miss the point that the ghosts aren't trying to wrestle you to the ground and eat your entrails, and that a grenade launcher wouldn't have any effect on them anyway so a camera is all you need. From the ghosts' perspective, you ARE packin'!
Finally, let me mention one game which I think IS truly terrifying, and is regularly overlooked on scary game lists (probably because of its frustrating design choices)- Siren/Forbidden Siren. I never actually played too much of this game, largely because it scared the living piss out of me to the point where I just couldn't go on (also because the puzzles were so damnnably obscure I felt I'd never beat it unless I practically played it with FAQ in hand, which kind of ruins the mood). The most nightmarish part is the enemies- the shibito. Basically Japanese zombies, the twist is that they cannot die. A lot of the time your character doesn't even have a weapon (since you control several different characters over the course of the story) but even when you do the shibito can only be stunned at best (although there is an exception when you shoot one causing it to topple into a well, although that's actually a highly obscure puzzle solution). Even worse, if you have a gun firing it DOES actually attract other shibito in the area, which can potentially make matters worse. Fighting is therefore not the answer (one of the things that actually turned me off the game was a mission where I had to be forced into combat with a shibito to proceed), but stealth (your usual recourse) is made difficult by the camera (which actually does a reasonable job of restricting you to what your character would be able to see, although a lot of it is technical problems). This is solved by the game's main gimmick- sightjacking, allowing you to locate the shibito by tuning into their vision to see where they're looking. Putting yourself behind the groaning, panting, bloodthirsty gaze of a zombie is a disturbing experience which can really freak you out, especially when you're jumping around between several of them, trying to work out the timing for you to creep from one piece of cover to the rest. Finally, the aforementioned fact that the game allows you to control several characters at once should make one thing self-evident- DON'T get too attached to them, the game can be very cruel...