When you’ve had a TV show around since 1963, with 752 episodes, 10 principle actors (with more Companions than I care to count), it’s expected to have a bit of turnover. 
Thankfully, Doctor Who built that into the show, and came up with the idea of ‘Regeneration.’ In short, the Doctor can chose to regenerate into a new body if he gets hurt enough (or dies). David Tennant announced he was leaving Doctor Who, and I worried, since my best friend only started watching the Donna Nobel season, and I was concerned she’d drop the show. She holds by the opinion that Donna is the best companion. I’m still for Sarah Jane Smith. It’s all about who you start watching with, I feel, and some of us accept a new Doctor more easily than others. The new Doctor, number eleven, will be Matt Smith, aged 26. He’s the youngest Doctor yet. The previous youngest was Peter Davidson, aka Doctor number five, cast at age 29, and the father to ‘Jenny’, who was created in “The Doctor’s Daughter” in Series 4 (that is, the Donna Nobel season). Davidson was my favorite Doctor, followed by Tom Baker, until David Tennant showed up, and now the order is Davidson, Tennant, Baker.
Seeing Matt Smith makes me think that we’ll get a lot of episodes about how this young punk can’t know what he’s on about. Tennant was ‘young’ and wild, but sort of with a perennial boyishness about him. Still, I always felt they were trying to make Tennant seem young. If they’re smart, they’ll play Smith as a bit less boingy and a bit more earnest. Tennant was a goof ball, Christopher Eccleston before him was a mood-swinging maniac. Smith should be neither. The humor value in having a 903-year-old in the body of a 26-year-old is appealing, but playing it up too much will be Tennant 2.0, and that’s always a bad thing. One of the joys of the Doctor is HOW different he is between regenerations. I hope they’ll swing round and get him out of the pinstripes, while we’re at it. Too Tennant, again. A knit sweater (no shirt, or a t-shirt we can’t see) and khakis, or dare I suggest jeans? Smith’s got a good mod/punk look to him, so pointy European shoes and a healthy does of arrogance may serve him well. Also a bike messenger bag would be appropriate.
Now, if SciFi would get off their duffs, stop airing Wrestling, and give me the Doctor Who specials and season two of Sarah Jane, I’ll be happy.