Just a few responses, firstly to TheNewGuy:
It's true that thieves do have trap selection, but the fact is the selection is often fairly cosmetic and not really practicable. The way thieves work is that you can only have two traps in existence at any one time.
THIS IS FINE AND I AM NOT SAYING THEY SHOULD BE ABLE TO SET MORE
But, the problem is because of this, they are severely limited to which traps they will ever actually use, because they can only set two. Anyone who has played a thief at 50 against even half-way alert opponents will know that you can hardly ever afford to open with anything other than antimagic, or maybe blindness, due to the way everyone can poison themselves now days.
It follows then, that your selection of 'flee' traps (traps you want your enemy to flee into) is also kind of limited. It is usually only feasible to set a big damage trap, or a lag trap. That is usually webcaster or chestbuster. The other traps (fireseed, poison needle, devil wheel, enfeebler, blackspore) are rarely, if ever, used because it is just not feasible to set one of them instead of another more effective trap.
So what do I foresee as the problem?
Basically, some traps combinations are so good that there is almost no reason, save novelty, to choose any other traps or to use any other traps that you may have selected. The {antimagic-backstab-flee into chestbuster} tactic is so effective that hardly anyone chooses a different combination, or uses a different method of PK than the one I just said. And why should they? It's so effective.
This is why I've always liked the idea of thief specialisation. I like the idea of having trap specialists, melee oriented, and more complete stealth oriented thieves. Maybe a path could be chosen at 20 or 30. Perhaps the trap specialists could have a wider variety of traps, some which required materials (starstone, gems, herbs) to be built, and which took longer to construct. The melee thieves would have very few traps, besides very basic ones, but would have better melee skills. The rogue path could have more of an emphasis on theft and escape, but would also be valid PKers in their own right.
THIS IS NOT SAYING THIEVES SHOULD BE MORE POWERFUL
I just think a little diversity would be good for the class.