Stargate Atlantis and Stargate Universe.

While Atlantis went for five seasons, the ending felt really rushed and it left some major things hanging. Maybe it's because they thought there would be a movie or two, as there was with Stargate SG-1, but no movies ended up being produced. The conclusion of the series felt rather unnatural. Unlike in SG-1, the situation with the primary antagonists wasn't really resolved, just... "brushed off", I guess is how I'll put it. It made sense, but left me thinking that there should have been more.

Stargate Universe had a tense, but mostly boring, first season. In the second half of its second season, it really picked up, and story-wise, it could easily have had at least a third season. I'm disappointed in the show's performance in terms of how many people watched it, but if it just could have continued with the kind of stuff that cropped up in its second season, I think its fortunes would have improved. 'Tis a shame it ended when it did. Unlike SG-1 and Atlantis, the ending didn't resolve anything at all. It just left its viewers thinking "well, I hope the characters do well in the future...."