The fact that it is basically half a game, some people didn't like Nero and it was easier than DMC3. These are he things that I see most people state as negatives, for me personally I liked it but I thought the fixed camera was terrible, it had way too much recycling and backtracking and I didn't like Dante.
A lot of people have said that they don't like the recycled levels.
Most seem to forget backtracking was always in DMC though.
....That's actually true, now that I think about it.
In DMC 1, you have to go back through the castle. In DMC 2, you have to go back through parts of Vie Du Marli. In DMC 3, you have to go through parts of Temen Ni Gru.
Then I think what people don't like is that they don't get to play Dante as much. It's mostly Nero. I would've like to be able to play as Dante more in DMC 4. --- Newer isn't always better. And I shall keep Dante's Force Edge polished and sharp till he comes back.