I've seen a couple of the things Smith's done, specifically the two Sally Lockhart adaptations the BBC did last year. He's really good, a bit more physically imposing than Tennant but has the same sort of intelligence, the same authority. He's got some serious theatre credits too (Starring with Christian Slater in Swimming with Sharks, part of the National Theatre's second cast for The History Boys) so he's got more acting chops than first appears.
Plus the dude looks ace in period clothing:)