I can see one problem immediately, though: Lag
With all that blood spraying around (and calculation of limb detachment) graphical quality would have to drop to an absolute low in order to circumvent that (Of course, that might actually help the game). I also have a couple small changes to add to the list:
Mood music: No soundtrack, a low drumming, soft violin or (quiet) ominous organ music (or other ominous but unobtrusive sounds/music) outside of combat. Metal starts the first time you hit something and fades out about 5-10 seconds after you stop hitting things. Loud music stops the blood sounds from grating on the nerves and the quietness sets the stage for it. A varied track of about 20 combat songs and 5 non-combat (they're going to be quiet and inftequent, so largely unnoticed) dodges monotony healthily.
Varied powerups: When you kill a certain amount of enemies (within a certain amount of time, say, 100 and each second not spent maiming the bar drops by 1), you will obtain a random powerup based on the level you have unlocked. Perhaps trees; there would be four ability trees each activated by a button on the D-pad. At 50 kills, you gain access to lv.1 powerups; at 100, lv.2; at 200, lv.3; at 400, lv.4, etc. The maximum level of powerup you can use from a tree can be bought with experience. This means that the problem with splatterhouse (the always-the-same-broken-powerup one) can be avoided.
No in-game collectibles for exploration: Running around takes away from the killing. If the scenery is special enough that you want players to explore it (or worse, they do) then you're not doing it right. Not for this kind of game, at least. Collectibles could be added for advancement, streches of powerup-free kills, high killstreaks and purchasing with experience.
Keep it going one way: Make sure the game can only be defined by one genre (in this case probably hack n' slash). Switching play styles can become annoying if done improperly and when people buy a game, that usually means they intend to buy one game, not two or more half-games mashed into one (exception: spore).
Customization: It shouldn't do much mechanically (with the exception of weapons), but that investment can add attechment. Starting players should have a wide range of equipment, but more can be gained through collectibles.
No death animations for the little ones: This is pretty straightforward; the time it takes to die should be directly related to the time it takes to be killed.
Less death punishment: Dieing while fighting a massive horde, only to have to run back through previous hordes in order to get to the same horde becomes frustrating. Respawn points between waves on easy, between combats on normal, between important sections on hard and between levels on extreme only.
Ixnay on the utscenecay: Lengthy cutscenes=annoying. The only reason you need them is for a game with a developed plot (and even then, cutscenes should be short, frequent and merged into gameplay), a game like this should not have a developed plot. Instead, short, infrequent sections to add humor and take a break from the endless gore can mix it up a bit.
No complex combos: wasted time and effort, especially when most enemies die in one hit.
That's really all that comes to mind right now.