In a somewhat tongue-in-cheek article, Solar Flare defines "Why British SciFi Television is Better Than American." Now, I'm aware this is generalizing terribly, and I don't necessarily think British SciFi is "better" though I think there are certain things that traditional the BBC has done better than American networks. Top of my list, and on Solar Flare's as well, is moral ambiguity. I'd add better dialogue to that list as well, and site Britian's theatrical tradition, from which their television emerged, as a reason.
But they also do certain things terribly - like letting really key points of the plot slip by at the same tonal level as everything else - and sound editing (David Tennant mumbles.)
Once upon a time, I used to think that the perfect film would be scripted by British writers and acted by British actors, but shot by an American crew, realized by an American effects house, and cut with an American editor's sense of pacing. (Hmm, that's pretty close to describing The Dark Knight.)
Today, of course, I don't think you can really separate them out like that. For example: Buffy was hugely influenced by the old Who, and the new Who was hugely influenced by Buffy. And everyone's watching and dissecting everything. Certainly, isolated episodes of both Battlestar Galactica and Doctor Who are as good as SciFi TV has ever been - but both shows suffer in the realization of their long arcs, and neither of them approach the overall level of some of the better HBO dramas when it comes to realizing "television as a novel." I found myself rushing through the current season of Doctor Who, for example, to get to Steve Moffat's episodes and then reluctant to watch any more past that. (More on that at another time, and how I'm starting to think that Doctor Who's biggest strength - that it can be anything anytime - is actually holding it back in the new model television.) By contrast, every single episode of USA's Burn Notice has been tremendous. As has every episode of AMC's Mad Men. Those two shows, btw, are my two current favorite shows on television. I haven't seen the SciFi television show that was this consistently good since the heydey of Next Generation (and I'm afraid to rewatch that in light of today's standards.) So something like "Blink" or "Silence in the Library / The Forest of the Dead" - as good as SciFi television has ever been. But I'm still waiting for the series that gets it right start to finish. And I'm not sure that, when it comes, is going to emerge from either British or American studio systems, old dogs slow to learn new tricks, but might be something that arises outside traditional channels altogether.
Meanwhile (and possibly pointing the way?), there's Dr. Horrible.