1.) What makes thieves "scary" (the more appropriate term is annoying) is the skill steal. Having two or even three entire thief paths doesn't make a thief overwhelmingly tough it just gives them a lot more utility. I mean honestly what combination of thief paths really worries you more then the steal skill. You can't remove the steal skill and have them still be thieves and what is annoying is that if you die in a PK there is a chance someone makes off with your gear. If you have to deal with a thief there is almost a garuntee that you will have to replace most of your inventory which is time consuming(especially prep bags) and it is significantly easier for a thief to steal from you then it is to be turned into a corpse in general. It is also frustrating because it can be near impossible to retaliate against a thief who is not interested in trying to kill you.

2.) I don't think role xp adds to actual in game role play in any way, shape, or form. However I also don't believe that it detracts from it at all either. My only complaint about rewards is a personal one that after like 14 years of playing I have never recieved anything beyond trivial rewards and I am frankly envious of players who get medium or significant rewards because I swear I have had to do at least one thing over the course of so much time that warranted an actually mechanically useful reward. That however is a different discussion.

3.) I don't get what you mean by game the system. I have no problem achieving 40K combined observe and explore xp before hitting hero with usually at least 30 kills in 60 hours. I like the explore XP system especially because I garuntee you that I wouldn't be trying to get buddies to explore high lords keep at rank 40 instead of waiting till hero when nothing agro's on me otherwise. It provides a solid incentive for taking cracks at tough areas before they get easier.