Everything. Just... everything. The main campaign more than the co-op, but the co-op does seem like it could be a blast. I am looking forward to the new puzzles, the humour, the clever new elements to the game and of course: GLaDos, she is a PG-13 SHODAN, but I love her all the same.
I am hoping they don't go overboard with the cake or companion cube jokes just to please the people who like going on about it, but I don't think I have that much to worry about, Valve don't tend to do silly things like that.
I am also looking forward to the challenge, I can beat Portal without thinking now, it takes me about an hour from the very start to the very finish.