Took me a week or so to really congeal my thoughts on this.
Matt Smith does not replace David Tennant, he is a totally new Doctor. One who is jittery, jumpy and always on the edge of something brilliant. He seems to be thinking far too fast for the rest of reality to keep up. He gets tongue tied because his brain has already thought five or six paragraphs ahead and Smith really conveys this sense of edge-of-seat life.
In two episodes, Smith steps up to stand beside all the previous Doctors and he stands as an equal, carving his own place in the mythos.
But what I really like about the debut episode is his new companion Amy Pond (Karen Gillan). She is like the best of all the previous companions but like the Smith Doctor, the Gillan Companion manages to live in her own place in the mythos.
We first meet her as the Doctor crashes into the world of a fiercely independent, latch-key 8 year old who just took in this stranger and accepted his weirdness as only a 8 year-old can.
When we meet her again, she is an independent 20-something who is still dealing with her "invisible friend" from her childhood. She is independent, still a bit vulnerable but who is making her way just fine in the world without help. She is still that 8 year-old, just trapped in the body of a 20 year old.
So when she goes to follow the Doctor, it is not out of love or admiration or anything else. She follows the Doctor because she has always followed the Doctor.
Her 8 year-old self comes back and she just jumps into the adventure because that is what fearless 8 year-olds do.
I really hope they do not turn her following him into some kind of romantic entanglement like they have the last few seasons. Her following is much more pure and adventuresome. She will follow the Doctor into the gates of Hell simply because he is the Doctor and she is still the 8 year-old who found this weird guy in her garden.
The perfect example is the quote from the trailers:
Guy: "He said not to follow him, what are you going to do?"
Amy: "Follow him."