Matt Smith started superbly... but that's it.
I don't think it's him, I think it's the fault of the material he's had to work with. The series' format and the arc-based storytelling has ill-served Smith. His Doctor has been superfluous for the last two seasons, often serving as the kindly wizard that takes Amy, Rory, and Clara places so they can have adventures. And when he has scenes to push his character, they often revolve around the mystery of River Song. Smith has been crowded out, and I think that's the great tragedy of his era.