The biggest sin I hear from many drummers: letting their hands (and feet) do their thinking for them - playing things just because they can, not because there is a musical purpose behind them. Playing a busy, fast fill, when something simple would be more effective, is one example of this.
(BTW, I hear this sort of thing from non-drummers, too - especially certain saxophone and guitar players...)
EDIT: I've been guilty of it, too...
