I don't know if there is an official changelog anywhere, but the general differences between Froyo and GB can be found online. (
example).
Also, I'm not sure if it was included with the official GB update for Thunderbolt or not (I'm rooted and running BAMF Forever), but one nice addition I like that isn't obvious at first is the inclusion of the "trace" keyboard, similar to swype.
To be honest, I've been running GB for so long now that I can't remember a lot of the differences between this release and the previous Froyo. A lot of it is stuff "under the hood", but there are also a lot of little improvements to the user interface as well.