Stepping away from D&D for a second. One of my favourite games that I haven't gotten to play yet (sigh) is 3:16 Carnage Beyond the Stars. In 3:16, you play Colonial Marines out to make the universe safe for Humanity by blowing the crap out of every alien you meet. It's not a terribly serious game.

There's more to it than that, but, that's the basic gist.
One thing I love in the system is each PC has a number of Strengths and Weaknesses. These are left blank at the start of the game and the character has a limited number of each. More can be gained through play, but, once you use either a Strength or a Weakness, it cannot be used again.
Using a Strength allows the player to declare the current contest over and that PC has won. So, you could spend a Strength in the middle of a firefight with xenomorphs and they'd all die and you win. To use a Strength, you have to detail some element of your history and then apply that to the current situation. If you're in dark tunnels a la Alien, maybe you grew up on a mining colony and you use your knowledge of mining techniques to plan and execute an ambush and collapse a cave on the bugs. That becomes part of your character from that part forward.
To use a weakness, it's basically the same thing, only this time, the player declares that his character has lost the conflict. The character is removed from the current conflict but, and this is the important part, the player gets to dictate the terms of that loss. Maybe he was captured by the aliens. Maybe he was medevac'd by emergency teams. Maybe the tunnel he was in collapsed, trapping him away from the firefight and he wandered his way back to the surface to rejoin the group. Whatever. The same thing applies here though that applies to the Strengths - you have to detail some element of your history that causes this weakness to manifest and remove you from play. Again, this becomes part of your character from then on.
What I find really interesting in games like this is the concept of shared narrative. The player is empowered specifically, in a very limited way, to be able to shape the story of the game in precisely the same way that the DM can do. Having read and played more than a few of these style of games, I find myself wanting to apply the same concepts to more traditional games.
If the player can come up with a plausible explanation, even if its one that I don't really buy, but, I can accept that he buys it, I'm pretty inclined to go with whatever the player wants. I have a player right now who is playing a Bard in a 4e campaign. Fun character. But, in the last session, they faced wraiths (note to self, NEVER use that creature again. - my first real experience in 4e grind. Yikes). The bardic abilities are skinned in such a way that the character is basically insulting the creatures to death.
Which, honestly, doesn't make a lick of sense to me. But, the player explained it thusly: while the specific ability says that X happens, don't get too hung up on it. Instead of the insult breaking the baddies brain, maybe it simply bolster's the bard's comrades to the point where they become stronger. Or maybe it's like that creature in Harry Potter, the Boggart, where if you laugh at it, it becomes helpless. There are a number of possibilities available if you don't get too tied into the specific description in the books.
Which, to be honest, is good enough for me.