I find it ironic that Depp, a man who turned down pretty-boy action movie roles (he turned down Speed before Keanu Reeves was offered the part) to avoid getting typecast, is now getting exactly that, he's like the go-to 'wacky guy' these days. I think he's a good actor, but needs to do more serious roles in my opinion. The Ninth Gate is probably my favourite Depp film because they guy actually gets a chance to act, opposite the superb Frank Langella.
Anyway, to the Doctor Who movie. Doctor Who has always and will always be a uniquely British franchise, there's just something about it (as with Hitchhiker's Guide) that can't be reproduced outside of Britain. If the world really needs a DW movie, then let's keep the current series cast, and bring in the big names to play unique roles we won't ever see again.
Matt Smith has done a fine job in the role, it really means something when you can take over from someone as beloved as David Tennant and not invoke much ire at all from fans.