I actually didn't know how to respond to this post at first because you said not to post the flaw but it seemed like you wanted the poster to post the flaw. And then I just realized you wanted the description of the monster itself without giving away what the flaw is!
So, enough of me being silly. Here's mine:
My monster is hunched over with darty bloodshot eyes and a trembling body. It's chest is cut open with the heart visible, bleeding profusely, unable to stop. It doesn't move however and whenever it chooses to (or is moved against its will) the heart beats much faster. The passage of time weakens this monster.
Ouch, I think that hit too close to home. But oh well, I gave it a shot.
Anyways, I this is a really good thread Chad and really gets the poster's creative juices flowing while acknowledging issues that may not have been brought up before.