Nothingness:
I think a minimum number of moves with no capture is a good rule to force a draw, if one of the opponents claims for it:
1) it is simple to remember for everyone
2) it helps preventing games to go on years, when only a player is very slow
3) if two skilled and defensive players agree on going on, it does not oblige to end up the game as a draw, if no opponent asks for it.

I think the most important thing here is to help games to go on faster.