Of course, there are the big differences There are the things that they really can't help, like the quality of the film itself and the special effects and alien costumes being silly (yes, I know, they were great for their time), but there are also stylistic choices that don't appeal to me. The relationships the Doctor has with his companions in the classic episodes, and this gets more prominent the further back you go, are more of a mentor/protégé(e) relationship as opposed to the friend/friend relationship that we see in the revival. There are some points that the classic Doctors are just plain mean to the companions, especially the female ones. As a girl, I cringe a little bit when I see it. It's probably more of a sign-of-the-times thing, but I'm not going to get in to gender politics now.
The element that really makes the difference for me is the personality and psychological makeup of the Doctor himself. He's a bit more stable in the classics. With a character of this nature, that has been redesigned literally from the inside out more than ten times, there is going to be huge changes from actor to actor. Perhaps it is that the art of television is evolving. The overall "strangeness" of each doctor is a character choice for each individual actor, but I think that the actors now feel more at liberty to go all-out with the Doctor's idiosyncratic behaviors. Is my theatre arts major showing?
Christopher Eccleston set the stage for this with his Doctor. Fresh off the front lines of The Last Great Time War, the ninth Doctor is drowning in survivor's guilt. With Tennant's Doctor, I'd go so far as to say that he's developed some serious PTSD, especially after the ordeal with Rose. His psychological state is still in one of decompensation, and it has to be if he is in fact going to become the Valeyard (and I really do want to see him become that), but he is more emotionally static as Matt Smith. Is my psych major showing?
None of this is to say that I don't like the classic episodes, because that's not true. While I favor the revival, I appreciate the classics.