Gotta have patience with this type of game.
Thief is *ALL* about stealth, you live it, you breath it, you think of it all the time in every step you tread... and you better tread carefully or they will spot you. The fascinating thing here is that the creators really did a great job in making stealth the most important factor, meaning that if you got to a situation that have been revealed and a fight begins you are most likely to loose, since you are rather weak. The main point here is NOT to be seen by any chance.
The weaponry you have is also quite simplistic, which makes it real good! You are a thief after all, so it makes sense that you will not be carrying a pistol, or any loud noise producing device, right ...??!?!
All you have is your trusty Black Jack - great for knocking these guards out cold (without having to kill them actually), your long sword - which you save for those most desperate situation when a fight is inevitable, your bow which is in fact your Swiss army knife, since as you proceed in the game you get different types of arrows which make the bow very useful for just about anything, killing from afar, coating noisy platforms with soft moss, extinguishing torches and more...
The graphics of the game are OK, no more. If it is graphics you seek then you may be a bit disappointed here. The NPC's and monsters limbs for example, look a lot like in the game System Shock 2 if some of you know it. It did not bother me however as I prefer gameplay over looks.
Sound is chilling and quite spooky especially when in a dungeon or a cave. Really good job there. You hear every footstep and you can "feel" that someone or something is getting closer and closer.
The game is quite stable too, never had any problems with it. I have played it on WinXP + SP1 with an old & trusty Geforce3. Don't know about the newer cards though...
One thing you must bear in mind is that you need a lot of patience with this game. This is no Serious Same and no Duke Nukem and no Splinter Cell. The gameplay here is truly unique and second to none. That's how it goes when most of the time you are actually hiding and looking over your shoulders...
I have never finished the game, I left it quite a while ago due to lack of time but the levels I did play were are great fun and very rewarding as you go along...!
This game is a must for every player who considers himself a serious gamer. At least give it a try :-)