Great series - I was passingly familiar with it before the revival, and I've really got into it since then. I like that Doctor Who's not afraid to push boundaries that more 'mainstream' sci-fi wouldn't dare go near. Sure it's sometimes pretty whimsical or far-fetched, but they get some damn fine stories out of it. A couple of people have mentioned the episode 'Blink', with the Weeping Angel assassins. The whole concept behind those beings is just so 'out there' and detached from our conception of reality, it was just a great idea in a great story.
Then, of course, there's the character stuff - which really drives the series across its numerous unrelated stories. It seems almost inevitable that Rose is going to reappear at some point, which should be interesting to say the least. But it'd also be nice to see more companions, too - in the old days, the Doctor sometimes had several companions of either gender, which makes for more of a mix than the recent 'one female companion' deal.
We're still waiting for the Catherine Tate episodes in my country, but hopefully it won't be long now.