[SixOfSpades]

The cheesiest things about traps in general:
1) They'll hit ANYTHING, not matter what the beast is protected from
2) They have such a HUGE trigger radius--A bad guy walks within 20 feet of a Trap, and his leg is bitten off.
3) You can lay all of them on top of each other in a big stack that will instantly kill anything in the game.

My suggested fixes:
1) Make Traps count as a +whatever weapon, based on the level of the Thief that set it, or perhaps on his Set Traps score. When setting each Trap, the Thief also has the option to make it count as a Normal weapon, in terms of what it can hit.
2) Cut the trigger radius by 75%, so the bad guy has to get within 5 feet of it to set it off. (They are triggered magically, you know, not by stepping on them. That's how they can hit Beholders and other flying enemies.)
3) Make it so that setting a 2nd Trap within the trigger radius of a Trap already in place will set off both Traps, doing damage to the Thief in the same manner as failing his Trap-setting roll would.

And get rid of the "You may not set any more traps in this area" thing. Fix #3 takes care of the cheese.