"The Bells of St. John" was a low budget episode
What!? Seriously, go back and watch Fear Her. That is a low budget episode. This was a blockbuster action episode with a fuck ton of money thrown at it.
I thought about this too while watching this episode. I thought back to Tom Baker saying he would do things that were decidedly un-human since the character is an alien. Smith made me think that with little things he does, tasting the leaf, reacting with disgust at the idea of snogging someone everyone alive would snog, and various other little "alien" actions.
Yup. It's gonna really hit it home with viewers in the anniversary special when they see Matt being all alien while Tennant flirts with Rose and references Eastenders or whatever.