...reversing Dante's personality transplant is a given.
Personally, I thought the bosses could've been better - hardly any strategy required or relevance or character to them. (And how cheap was Argosax?) If they'd put a bit more thought into those and how to make the fights engaging rather than tedious, etc.
In DMC1 you have bosses which are at least in some way (however loosely) connected to the plot and require a particular approach to deal with. Nelo Angelo and Mundus' appearances go without saying. Nightmare, while a bit random, is probably one of the most interesting bosses in the series with its variety of attacks and the fact there's more than one way to kill it, yadda yadda yadda. In DMC3 and 4 we've got the plot hinging on several boss fights that keep up the tension, with only a couple that seem out of place and a bit lacking in fun (gigapede, for example).
DMC2 had quite a few bosses with no particular purpose to them and hardly any strategy required but hack, hack, hack, dodge/jump. The tank? The big floating skyscraper head thing? Freki, Geri and Bolverk? Furiataurus, while at least requiring you to run around a bit, is a pointless grunt. Phantom makes a meaningless cameo and doesn't even mock you. And so on. If the game itself had a stronger plot in the first half with the bosses worked into it a little better, it might have been a more enjoyable experience. The randomness of the areas you visit is disorienting, as well. I much preferred visiting Mallet or Temen-Ni-Gru for their consistency than I did Dumary, jumping from Mediterraneanesque villages to skyscrapers to creepy gardens to oil rigs to airplane hangars...
Don't get me wrong, DMC2 is the first of the series I played and I must have enjoyed it enough to risk buying the original DMC and every other game afterward. After playing the rest though, 2 feels like a trashy, lazy and disjointed installment. What I like most about the series are the characters, story and the gothic atmosphere; they hold the whole thing together like glue, which is pretty much what 2 lacks. Chi's story should probably have been expanded a bit more as well.