There are times when I've hesitated to start watching a new show

Heh. Yeah, me too. I tend to creep up to new shows sideways, trying not to jinx them. But I couldn't help myself with Rubicon--a couple of friends I trust recommended it, and I'd heard something good about its deliberate pacing, and then I just fell. I BLAME MYSELF. *g*

In terms of short runs, where do you think the fannish tipping point lies between "too short" and "just long enough" to gain support a sustainable fannish base?

That's an interesting question. But I've never quite been able to pin down why something becomes a widespread fannish darling and something else doesn't. I mean, I sometimes look backward and hypothesize about possible reasons (e.g., why Man from UNCLE ended up huge in fandom while I Spy didn't), but of course, looking backward I can always connect possible dots without knowing whether they were really the cause.

Now and Again is a great example of my lack of predictive power--I don't really know why it never took off. Interesting and flexible premise, very handsome leads, solid potential for slash and for het...and yet, fannish production is extremely scarce. I didn't see that one coming! Very likely, as you say, that the number of episodes is not necessarily a strong factor at all compared to other factors in play (consider something like the BBC's Sherlock, with only three installments to start with...although, granted, they're longer than the typical TV episode).

Rubicon, I suppose, will just have to settle in with the rest of my beloved-single-seasons (e.g. Peacemakers, sigh). I guess I don't see it belatedly taking off among many other people to keep us few company--but I can't help but love it and feel fannish about it, no matter how lonely a position that is. Alas.