Wander Over Yonder - Though the second series wasn't as good as the first, it was still an enjoyable show, and I'm sad to see it go.
Atlantis - This was cancelled purly because it cost to much to make. I just wish they'd cancelled it before writing & filming the ending to series two. Or, you know, cut out the last five minutes or so and make it more of an actual ending, instead of a semi-cliffhanger lead in to the none-existent series three. :/
Sinbad - Though this did tie up the main plot, there were still many unanswered question and lose ends, plus an obvious sequel hook, but it was canned after one series. Most annoyingly was that the official reasoning was that its "story has now been told", when it obviously was meant to have a second series.