I actually prefer closed canon (in both books and TV shows) because then either the canon's turned out the way you wanted or it hasn't, but at least you know the worst. There's no horrible suspense (I'm a wimp about suspense, both narrative suspense and meta-suspense about the narrative) and worrying about whether it might get cancelled with no time to wrap up loose ends like Firefly or, worse, keep going and going but jump the shark so that watching is painful but you feel guilty about stopping. Er, not that I went to all three Star Wars prequels out of that exact feeling of guilt or anything.

It helps if there were some satisfying plot arcs and a sense of closure, I think - it makes watching the DVDs more of a short, sweet pleasure than a sharp letdown. You can pretend it was always intended to be a small canon, rather than one cruelly cut off in its prime. Also, short canons are much easier for the lazy to research.