7. Youthfulness
Some people will tell you that the Doctor must be aged 35-50, because younger actors are incapable of playing thousand-year-old aliens. Personally, I don't there should be an age-limit on the Doctor, and Matt's proved that, but by only being 26, he's proved another thing: the Doctor is as much about communicating to children as it is adults. And Matt Smith can connect to a young audience, not just because he's young, but because he's youthful. Sure, he's extremely merchandisable, but he acts his young age and having such a youthful actor in the lead role has kept the role fresh. Sometimes it's nice to forget that the Doctor is a grandfather and just escape into the drama of running from Daleks, and for a younger audience, it's nice to imagine him as your babysitter. It's probably one of the reasons an older Twelfth Doctor was cast; Matt Smith has defined the role for any young actor, and it's time to take the Doctor somewhere a little more responsible though. I have to say, I'll miss the childishness though.