If you want to stick with how it's done in the original Thief games, then it's indeed rather a difference in loot count, additional objectives, no kill objectives, sometimes even no knockout objectives, rather than raising the overall difficulty with the difficulty settings. But, I think I prefer it that way anyway. I always play on Expert, and, the original missions are all absolutely doable, as are other fan missions.
I can only repeat myself: The problem is the general difficulty with t...