People say Dead Space 2 is 'more of a shooter' than the original. I dunno about that; I just finished playing both relatively close to one another and they seem like the exact same thing to me. There is one major difference: I found the original to be okay-ish while I had a pretty lousy time playing the sequel.
Enemies feel faster and more aggressive than before, an exaggeration of something I already thought was bad about the first game. The sequel is very eager to spawn dangerous monsters behind or literally on top of you while you deal with intentionally clunky controls and low resources. Some may refer to these things as, "survival horror elements." I just thought they were irritating. You're often thrown into relatively large arena-style rooms where enemies swarm at you from all directions, and it is always a disaster. An odd change from the original is that monsters no longer drop resources when dead until you stomp or shoot their corpse. I suppose they thought stomping stuff was cool (they were partially right) and wanted to encourage that behavior. It just makes already clumsy battles even more bothersome.
New to the sequel, hero Isaac Clarke now has a face and a voice. While you might claim that not having those things increased the feeling of isolation and spookiness in the first game, Dead Space 2 proves that the added emotional expression is worth it with lines like:
"Fuck!" - Issac seeing a bad thing.
"FUCK!" - Issac panicking from a very bad thing.
"Fuck." - Issac, resigned to his fate.
"I'm on it." - Issac, informing someone that he is on it.
Oddly enough, a love interest comes into play about halfway through (even though he's still obsessed with his dead girlfriend) and the whole thing takes a weird turn to Body Horror Uncharted. The action scenes become more over the top and the two engage in cute banter (cute banter is the only way video games express relationships, even in the middle of a hellish, apocalyptic scenario). Issac starts spouting Harrison Ford lines like, "Didn't I tell you? I'm FULL of bad ideas!" It's just a strange conflict in tone all around.
They did improve the zero-gravity sections. Instead of leaping from surface to surface you now have full flight control. They also heavily reduced the amount of gravity gun puzzles, and the ones that remain are much less intrusive. Unfortunately, it wasn't enough to save this one for me. I just didn't like it.