Besides the covers, what differences are there between the US and UK versions of the Harry Potter books?

There is no difference in plot or characters. It's just that some British terms were changed their American equivalents so that American readers could understand them (e.g. Philosopher's to Sorcerer's, jumpers to sweaters, dustbins to trashcans, Sherbet lemon to Lemon drop, mum to mom), and some words were spelled differently. The American version also has small illustrations at the beginning of each chapter, and the typeface is slightly larger.