Legolas. Lots of Legolas. And doing that thing to get on the horse in motion, and reacting to deaths so heartbreakingly...
Merry and Pippin being much more proactive to get the Ents involved in the war. It was much more of a passive thing in the books.
Haldir's death. I actually cried during that slow-motion bit, and then Aragorn running up just in time for Haldir to die in his arms... *sobs* I hardly ever cry at movies, but I was teary from that point on.
Slashiness! Much slashiness!
Gollum was simply amazing. If Andy Serkis and the animators don't at least get nominated for Oscars, there is no justice in the Academy.
But why was Faramir evil? Wasn't the point supposed to be that he succeeds where Boromir failed? That he passes the test? And I don't think it was made clear enough that the Eowyn/Aragorn thing is entirely one-sided on her part - that although he respects her and cares for her, he doesn't love her, and he never will.
Those were the only things that really bugged me. I didn't mind the Elves being at Helm's Deep, because it mirrored the first Alliance, and also because it allowed PJ to kill off a recognisable character (and I think that was important, to get the cost across to the audience), and also to show that even Elves can die. The first movie didn't make that point, because it was concentrating on Frodo. This one was about the world at large.
In short, despite a couple of gripes, I loved the movie as much as I did the first one. I want to see it again, as soon as possible. I'm sure there's things I missed.