I've finished the campaign mode now, Angela. I really liked it and, in almost every way, I found it to be an improvement over the original and a worthy sequel. There are some elements from 1 that are totally absent from 2 and, while I missed them, I kind of liked that they were willing to let stuff go and it means the first game is not obsolete.
The first act of 2 is very much the same as the original but, as things go on, it just gets bigger and better. I adored the last couple of acts, though the very end left me just a little unsatisfied.
In terms of narrative, yeah, I feel that's their weakest point. To me, it's not a case that they set up questions for the third game. It felt more like they just sort of threw whatever they liked in at the time because it felt cool. I would very much doubt that, right now, there are any answers to the questions. Stuff is introduced and then forgotten about several minutes later and there are inconsistencies all over.
It doesn't make the game bad but I think a lean, clear narrative would have made the game even better.
But for plain fun, the game totally wins for me. And there are some real improvements here - especially with the friendly AI. I got bored very quickly of my teammates commiting suicide while I had to stare at the red circles and take on the enemies on my own. Dom in 2 can kick some ass quite well and that felt better to me. The improvements to the visuals also aid gameplay as the problem I had in 1 of not really being able to tell friend from enemy in the heat of battle is completely absent here.
And Horde mode is a really fun addition and something that will keep me playing even if I don't get into the online thing (which I didn't for 1).