Interested to hear your thoughts. Some of our favorite games can be completed in about 2-3 hours (or maybe 4-5). I personally love these "one shot" games that can be played in one to two sessions. What is it about a great short game that makes it truly great? Does the game have to move briskly? If it has a story, how is it paced any differently than a longer game? How do gameplay elements have to progress to make the game feel truly satisfying?
Some examples of shorter games that I think really do a good job of compacting a great experience into a short amount of time:
VVVVVV To the Moon Portal Limbo
I guess we can talk about older games as well, but I feel many older games that can be considered "short" really only become short after we've mastered them. The original Zelda, I can beat in under 5 hours probably, but that's really only because I know that game like the back of my hand.
As some have stated, having a clear endgame seems really important with these games. What do you want the player to do? What do you want them to have experienced by the end? Is what you had planned enough, or do they need more in order to fully experience what you had intended?