I.e. the plot wasn't full developed, it was crammed too much, it left way too many questions unanswered, it left it wide open for a yet to be seen sequel, etc.
Akira
Yes this movied kicked alot of ass, but it was just too short for me. The story it was based off (a good 2,000 page or so manga) had plenty of story to tell, but it was choped, crammed and pureed into only 2 hours.
I would definately love to see a 26 epside series of this. Hell, even a 13 episode series would be well worth it.
Spriggan
The interesting thing about this was that the movie is actually based off vol2 (of 11) of the manga. So theoretically, a tv series, another movie, or even an OAV can still be adapted from the manga sa there's still plenty of story.
Plus McDougal's last line: "This isn't the only site like this. We've discoverd hundreds around the globe. You'll never stop them all..." or soemthing along that line.
Hades Project Zeorymer
Good series, but they took a story that could really fill a 13 episode series easily and crammed it into 4. Everything happens so fast you get dizzy. Talk about a forced situation.
Hand Maid May
Even though it had an 11 episode run, it just didn't feel complete. I don't know why, but it could use another season.
Jungle de Ikou
Beyond all the fan-service, there actually was an interesting story brewing. They even left it super wide open for a sequel. In fact, I don't know what kind of ending that screams sequel more than this except just about every movie being made nowadays.

Outlaw Star
Though it may have run a good 26 episodes, the ending seemed to hint at a possible sequel. I'm still waiting.
Ranma 1/2
Yeah, yeah, I know, but more Ranma is never a bad thing.
Hellsing
It's been said a million times, but a second season, or a more faithful manga based version is desperatley desired.
Sonic
Another Sonic movie/OAV/whatever would be great. And it should be from the animators who did the first one and Sonic CD's animated sequences. Sonic X looks OK, but it doesn't seem to do Sonic justice.