I agree with Blass to a point... but at the same time, there was definitely new stuff in The Dark Knight score that you couldn't find anywhere in Batman Begins (i.e. Joker/Harvey themes). Since neither of those characters are returning, and we are meeting Selina, Bane and Miranda/? for the first time, I think we can expect new thematic additions.
There is a considerable difference between the soundtrack for Batman Begins and The Dark Knight. The score for BB was very subtle, and each track was named after a species of bat. There weren't really any melodies you could whistle or play with a single instrument that would be instantly identifiable as "from that Batman movie." When I say "identifiable" I mean to the extent that some of John Williams' more famous scores are instantly identifiable. I don't think that its necessarily a bad thing for there to be subtlety, because some might argue that over the top themes can overshadow the other film elements. I mean, take away the John Williams score from Star Wars, and you destroy the franchise. While the music from Batman Begins definitely adds to the experience, it doesn't define it necessarily.
With the Dark Knight, you have track names that actually hint at certain plot points. From the first track, "Why So Serious?" all the way to the end, we are treated to a harmonic fusion of classical and contemporary styles. I think Zimmer/Howard did an excellent job creating new themes for the Joker and Harvey. Combined with more developed themes for Bruce/Batman, as well as some sentimental moments involving Rachel, it is a very well rounded score that stands up there with the best of them.
Richard Wagner, the famous opera writer and composer from 19th century Germany, more or less created the concept of "the motif" with his Ring of the Nibelung opera cycle. John Williams and others in our time have captured that concept and developed it to a great deal. Everyone knows the basic Star Wars theme, for instance. But if you ever listen to the broader score for Star Wars, you find that each character or group of characters has a theme or motif that plays during their important scenes. This concept is executed masterfully at the end of Revenge of the Sith, which, although a mediocre film at best, presents us with one of the best examples of fusing different themes and motifs from across a greater body of work into one ensemble piece.
Hans Zimmer is a much more subtle composer than John Williams. However, I think that he uses the concept of the motif much more than John Newton Howard does in his work. So in TDKR, I am expecting the Batman/Bruce motifs to reach a zenith in their evolution. Further, I am expecting new motifs for Bane, Selina, and several of the other characters in the film. I am also expecting some minor recycling of the Joker/Harvey motifs from The Dark Knight.
I know most of you hate reading long posts. But musical analysis tends to be on the more collegiate end of the chat forum spectrum, so I expect a bit less pithiness when I read threads like these.