Most of the new stuff could be seen as simply extensions of what was already there in the theatrical release. There's some additional exposition that helps us see why Michael is in that city, and allows Selene to sympathize with him some more. There are actually several new bits that really help a lot to flesh out how Erika (the blonde vampire girl) feels, and why she helps Selene escape the mansion (which was certainly possible to see in the original, but the new material helps a lot).
Otherwise, mostly little things here and there, as I recall.
As far as extras, they have a whole new commentary track, with Len Wiseman (the director), Kate Beckinsale (Selene), and Scott Speedman (Michael). This commentary replaces the one found on the first DVD release.
Hope that helps,
Armin