Well, in my Fantasy the Mages are an unlimited source of magic: They can fly at hypersonic speeds without effort, make castles and countries appear out of nowhere and throw one magical nuke after another without a care in the world... I don't care at all to explain anything from a logical or scientific point of view, and reading what everyone is writing in this thread about magic is a very strange and sometimes amusing experience for me...In my world magic always has a price. The energy has to come from somewhere. I see a wizard as more of a channel for magical energy than as a huge magical battery. And magical energy isn't limitless.
So Harry Potter, or D&D style magical combat is simply not possible. The kind of energy for those battles simply wouldn't be available.
You all would read my works, shake your heads in desbelief and say "that's impossible" over and over again, and I know that's bad for Fantasy stories because most readers want everything to be realistic... but now I am happy with it!! I love my Fantasy anyway =)