"You are the only mystery worth solving."
One of the key features of Doctor Who that makes it such a great show is the relationship that the Doctor has with his companions. There has been romance, both requited and unrequited, amazing friendships, rivalry and even a huge complicated familial relation, and I'm only speaking about the new Doctor Who series.
So what kind of a relationship are the Doctor and Clara Oswin Oswald going to have? If reports are to believed, they apparently have a Spencer Tracy-Katherine Hepburn dynamic, which is a good dynamic to have. He is clearly intrigued by her, and she seems to have some great cosmic connection to him which is why she has found him in two episodes and in the prequel to "The Bells of St. John", the latter as a child. Now I know that the internet is irritated with the whole Doctor meeting his companions as children and then making out with them as grown-ups thing, but it was a rather adorable prequel still.
As pointed out in my Clara post, there are several theories flying about who she really is, and this will obviously reflect on the relationship she has with the Doctor. I, for one, really want her to meet River because that would be a fun thing to watch :P
Also, I am already loving Matt Smith and Jenna-Louise Coleman together in interviews and stuff. They are going to be excellent!