So what does everyone think about demos, and what makes the good ones stand out.

You have to know whats going on, not just dropped into the middle of something. 30 awesome minutes is better than a decent hour too. Doesnt have to be long.

Has to be long.

A good demo gives an idea of what the gameplay and game will be like, not the opening sequence you've seen upteen million times on blogs sites and youtube. It's a snapshot of the game - doesn't have too be long, say anywhere from 10 - 20 minutes is good.

IMHO.