Well, even though I've never played it, I've gathered enough from story summaries and others who have played it to know two very important things:

(SPOILER)-Roxas is Sora's Nobody; he was created when Sora turned into a Heartless for about 10 minutes during the first game. (His heart split in two or something like that.)

-You didn't fight Ansem in the first game, that was Xehanort. (One of Ansem's students or something. Must be obsessed with darkness... )

I'm sure there are many others though, KHII looks like a much longer game than the first. (Which is good, I thought the first was kinda short.)