I don't think Daniel is that bad. Ok, he is probably not the best actor ever and he will never recieve an Oscar for his role as HP but he is pretty good on the stage. I heard a lot of people that were impressed by his performance in London (currently in NYC) and he seems to be a better actor on stage than in movies. Maybe he discovered his talent just after he signed the contract for 7 HP movies.
Some actors are similar, e.g. Josh Hartnet. He is a brilliant actor in movies (love him in Pearl Harbour and Black Hawke Down) but his stage performances are poor. I've seen him in Rain Man and was a bit disappointed though. I think he should consider to go back to LA and work with American movie producers instead of English stage directors.