Since I am presently deep in the White Box, I had a thought...
Some bloggers (me included) have advocated introducing a saving throw-based magic system for magic-users.
Well, why not do the same for thieves? When a thief does something "thiefy" (DM's discretion, or outline the specific abilities like move silently, etc) the player makes a saving throw to pull ti off successfully. Optional rule: Dex modifier affects the save. Optional-optional rule: Elves have a +1 bonus to move silently/hide in shadows and a -1 bonus to open locks and disarm traps. Dwarves have a +1 to open locks and disarm traps and a -2 to climb walls. Halflings have a +1 to hide in shadows and pick pockets and a -2 penalty to climb walls.