If you were familiar with DW's legacy, you'd know that it's part of the show that the actor changes from time to time. This accomplishes three goods: It's a neat plot device, the actors are less threatened by typecasting, and young new actors have a chance for something quite big. It also keeps the show fresh. On top of that, if changing the actor wasn't possible, we'd have no show no more since 1975 at latest (that's when the first Doctor's actor died).
Sure, Tennant was awesome, no question at all! But he once was new too, and had to acclimate and develop. He too wasn't instantly incredible, and from what I have seen, Smith made a more than solid start. But nothing can help personal preference. I'd like to give actors more chances than 3 episodes before I dismiss them though (Look how stiff and strange Patrick Steward was in the first Star Trek: TNG episodes. He ended up as THE icon of starship captains.)