It may be a nice feature if game challenges are automatically declined, if they are taking up your last game slot and would prevent you from starting a game.
Example:
Player A sends me a challenge and this occupies my last free slot. I want to play against player B by joining his game. MArcomage would now automatically decline my oldest pending challenge to allow me to start the game immediately.
An option to disable this might be nice, too. This is certainly no biggie, but might be worth doing if you've got the time :]