The enemies in 2 are better, they're more varied and travel in packs. I'm also more motivated, I need the Big Sister to survive. In 1 I just did shit because a guy I never met asks me kindly to do stuff. The weapons and plasmids are stronger, especially those plasmid upgrades that yahtzee ignored due to ignoring adam. And the only reason he didn't need them is because he didn't take up any gathering. You can still be evil after you let little sisters gather adam, if you don't you just get a lot less adam. So the choices are better too in that respect, as well as the fact that you get to decide if you'll assist in suicide, kill somebody who's got a grudge against you but is otherwise fine, and whether or not to kill somebody who's responsible for your fate. Also you get to dual wield shit and not have a horrible ending in 2.
I'd also like to mention that the prototype big daddy is better than the other's because he was too expensive to make on a large scale.
Oh, don't get me wrong by the way. 1 is still scores better than 2 in pretty much every field except gameplay, which I feel 1 get's trounced in.