Originally Answered: David Tennant fans: what makes you think Matt Smith makes a bad Doctor Who?
Matt Smith isn't a bad Doctor, but he had pretty big shoes to fill. David Tennant raised the standard pretty high, and Matt Smith didn't really live up to it. In some ways Matt Smith is a better Doctor, he looks more alien-like, has a better sense of humour, but in general, the David Tennant episodes were better. There wasn't a long running continuity that a first-time watcher wouldn't understand, the aliens and scripts were also better. In a way, Matt Smith's alienish appearance failed him also, since David Tennant's Doctor could easily fit into normal society due to appearance, but Matt Smith's Doctor would stand out a little bit. So, overall, Matt Smith is not a bad a doctor, but David Tennant set the standard pretty high. (Hoped this helped).