One design goal that I've seen pretty be pretty successful is to get players into a tight retry loop.
It's very disheartening as a gamer to be playing something, lose, and then have to go through a lot of rigamarole to get back to where they were. "Just one more time" isn't really what people think if it takes them a few minutes to get back to the fun stuff.
Some examples in various genres that try to get the player back into the game quickly:
- "rewind time" in games like Dirt 2/Grid, or Prince of Persia: Sands of Time
- near-instant retry in games like Canabalt
- quicksaves that are frequent and generous in pretty much most modern games
I realize this is sort of meta-level on the fun factor, but it does help keep people glued to the game for a longer period of time.