I realize The Tick wasn't very good...at ALL! But I feel it had SO much potential, it was just never properly developed.
Though not sci-fi, I will never forgive them for canceling Clerks, the animated cartoon. It was awesome!
Gotta say Farscape. I wish it had been on HBO or something instead of SciFi, I think it could have gone even further than it did. At least it got the miniseries so it didn't end on that ridiculous cliff hanger, but still. :-/