The only complaint about this game I have so far has to do with the skilltrees,in BL you could choose better comabtrifle and shotgun handling for the Soldier or you could skill on his turret to be better or play as a medic, for the siren it was SMG and elemantary damage, for the berserker it was melee and rocketlauncher damage and the Mordecai character was specialized on sniper rifles and Revolver.
I don't see much of this in BL2, the assassin still has something like a Sniper rifle skilltree but I don't really know about the others, the gunzerker has a skilltree for overall weapon handling and gunzerking but I on't know how this will keep me playing since I can use almost every weapon with 1 character as god as with the second.
So they don't play that differently IMO besides their actionskill, you people can still prove me wrong.
For the most part, they're still very similar just much more refined.
Maya is THE support class. Lilith was a support class, but Roland's turret healing/buffing people, and him being given the ability to heal people by shooting them made him generally better at being a support class than her. Maya on the other hand really is the healer and supporter of the group, versus Axton which is still "Support" but a much more offensive one than Maya. His turrets deal great damage, and can be a good distraction which is invaluable sometimes.
Zer0 is great because he's got Mordecai's sniper abilities, but still has the ability to be a great hit-and-run guy, which sorta takes what Mordecai was and expands him.
Then finally you have Salvador which is the tank of the group like Brick, just doesn't have to always be up close and personal. The dual wielding allows you to be further than Brick (Considering his special was bringing out his fists) but you can't zoom in so you still have big accuracy problems, which means sometimes you have to get up close and personal.
The only two I really know right now though are the Axton and Maya, because that's who Shwah and I are for our playthrough. The rest is what I've sort of gathered from what people have told me.