I'm in the minority that I'm not a huge fan of David Tennant's Doctor. I think he jumps around and chews the scenery far too much, although I think he worked very well with Catherine Tate's Donna. Even acknowleding his popularity, though, I think it's good that he's not overstaying his welcome. Tom Baker stayed far too long, and ratings were in the doldrums and the show was largely forgotten when he finally went, a blow from which the show never really recovered.
I also think it's a good idea to let the new exec producer to start crafting the show in his own style right from the beginning.
All that said, I'm surprised he didn't stay for one last season in 2010. It's a little strange to hit the "regeneration" tease so hard a few months ago, and then turn around and do it for real just a year later. Hopefully it won't take some of the excitement from the real thing when it happens.