I am a big fan of the Thief series and have played it since I purchased Thief Gold. I wasn't quite there from the beginning, but this is a great game. I was amazed at what could be done with an FPS engine, particularly after Doom, Heretic, Marathon, Unreal Tournament, Quake, etc. seemed to be these stale puzzle solving and shoot the hell out of everything that moved games. I remember a couple of rooms in Doom II that had what seemed like billions of monsters, of course they were all one pixel thick. Thief didn't suffer from this problem at all, it was rich graphically and narratively.
Ever since, I have been a fan of the "sneaker" game. I love the Solid Snake series, and the Splinter Cell games for the Xbox. Neither of these games would probably exist without thief. There is a difference between these and Thief, namely that they are 3rd person view and Thief is 1st person (a difference I prefer), but I too am looking greatly forward to the new Thief release.
Thief was one of those games where it was actually fun to watch someone else play. I used to watch my friend Josh (also a friend of Bob's) play this game for hours. Great stuff.