Trying to tell a human story in that short a time is problematic. There's no time for character development so you have to either go in with a fully-stereotyped character or do something other than a character story, and either one is risky.
I've thought about doing a trailer and just not dealing with complete narrative storytelling, since that certainly frees up a lot. It allows for a lot of shorthand, but there's still a lot of disconnected scenes that need to be shot.
And I really don't need an ending. :)