Forgot KOTH was cancelled a fucking travesty on FOX's part considering it was replaced by Napolean Dynamite which was a fucking embarrassement. It was also replaced for Cleveland Show, but I actually enjoy that one.
The Critic. That show only lasting 23 episodes is a crime.
Sarah Connor Chronicles. Did you see the cliffhanger that they ended on? Jesus Christ!
Thundercats 2011 = Wanted that reboot since I was 7 years old and just as they were building shit up and getting hardcore, NOPE! No more episodes. Ugh.
Arrested Development = Well, at least it got an ending, but that show deserved so much more success than it got initially.
Firefly = I will never forgive myself for not watching that show when it was out.
Clerks Animated Series = It got shit on by Kevin Smith fans for not being as hardcore as his movies (how could it be?) and the networked fucked around with it and basically designed it to fail. Well, only 6 episodes were produced and they were fucking awesome! That show deserved to survive.
Mission Hill = That one was just on the wrong network when it came out. When its reruns got to Adult Swim, I can't tell you how many people I talked to that saw it for the first time and had never heard of it previously. Hell I was one of them. But great show.