Everything from attacks of opportunity to the way armor class works now is new and different. You should definitely give Combat at least a couple of readings before trying to run your first game. One of the early concerns about 3E that was voiced by some was that it was being designed to 'require' miniatures. While I don't think that is true, the game is now certainly very miniatures friendly. Everything from certain Feats to attacks of opportunity requires a good knowledge of exactly where the PC's are in relationship to their adversaries and of course the easiest way to do this is with minis Magic has gotten an overhaul and in my opinion it was definitely for the better.
Divine spells are also divided up by domain.
Each deity grants a cleric access to two domains, which roughly correspond to the old major and minor spheres. Access to a domain also grants the cleric certain powers. For instance, a cleric with access to the domain of Fire can turn and destroy water creatures as if they were undead. In addition to his two chosen domains, a cleric also has access to the list of 'generic' spells that anyone may cast. There is a wealth of other new info in the Magic chapter.