Dead Space 3 was a good game. Why don't people bitch about Resident Evil 4 or Resident Evil 5 as there's nothing but constant shooting in them and little else?
If you get through more than 10-15 minutes of the game, Dead Space 2 was actually pretty shooter heavy and everyone praised as such a remarkable game, Dead Space 3 is only a bit more of a shooter than Dead Space 2 is. Co op was added to the mix so people could play two player. Heaven forbid Visceral allowing players to enjoy a great game with a friend.
People complain about RE5 all the time; not sure what planet you've been on if you've missed that train.
As for RE 4, people enjoy it because it actually attempted to have some semblance of a horror atmosphere and pacing. Dead Space 3 doesn't even pretend to be a horror game. Ammunition is anywhere and everywhere, so there's never any sense of tension knowing that whatever happens to stroll into the room next can simply be blown apart with no exception. And while Dead Space 2 (which started the awful trend towards shooterdom) barely ever tried to scare anyone outside of jump scares, Dead Space 3 won't even do that; the monsters just all walk into the room with the "music" blaring.
Long story short, there's no sense of atmosphere or pacing. Everything's been streamlined, and there's nothing in the game that's remotely tense. Co-op is honestly the least of my complaints, since it's optional.