Barette, I definately like the idea of leveling up on monsters/npcs. Items that can only be found through stealing, however, I dislike. Realistically, if someone has something you can steal, you can kill them and take it as well. Also, a prime concern is game balance. Giving the thief more power, like this, would be unfair to some other classess. If you put in steal only items, you would have to put in kill only items as well. But then the thief backstabs the monster after stealing from it, and gets both.
One concern I have about npc stealing, is if a thief goes through a part of a dungeon using stealth and such, and steals things off every monster there. Then a group comes through, kills a lot of stuff, and gets nothing for their effort. This could get very frustrating, especially if the monsters are difficult.
As for your idea about \"safely\" stealing when you\'re a high level. If thieves could steal in safety at high levels, then a high level thief would be unstoppable. There will always have to be at least a significant risk to stealing.
Another idea, asside from random odds based solely on skills, could be the ammount of time the thief has spent watching the target. If a thief follows someone arround for a long time, they\'ll have better odds of stealing, but they\'ll also be more likely to be seen by the target. And as your level goes up, you will have to spend less time watching your target. I think it could be a lot of fun trying to follow someone arround the city, while avoiding being seen. Or, for higher level thiefs, they can watch/follow the mark from a greater distance.
Just some thoughts. Keep it up!