Thief: The Dark Project

The Good
1. Complex AI. Guards will chase you, will look for you if you give them the slip, and will get bored of looking if they can't find you. Dead bodies alert them to your presence, so hiding said bodies becomes a priority.

1. Open-ended gameplay. This is no railroad shooter. You must come up with your own way to achieve your objectives. Do you try a frontal assault on the Manor? Quite possible, but not advisable. Sneak in through the sewers? Maybe a rope arrow placed right there? Kill the guard? Knock him out? Distract him? There are a million ways to play the game, including the ever popular "never let them realize you're there."

2. Difficulty levels that are meaningful. Not merely content with giving you less health, higher difficulty levels mean more objectives and limitations on how you can go about it ("You're a thief, not a murderer - don't kill anyone"). You haven't played the ENTIRE game unless you've done it on Expert.

3. A unique inventory. Your bow has several types of arrows, each with a different purpose. Water arrows to douse torches (making you harder to find). Rope arrows for climbing to interesting places. Fire arrows for when something absolutely MUST go boom. Your other tools include flashbombs, lockpicks, mines, and holy water. It's up to you to use them well.

4. An amazing storyline. Set in a rich steampunk setting, you are a thief caught up in a horrible plot to destroy civilization. Which puts you in the last position any self-respecting thief wants to be: the role of hero. The voice-acting is flawless, the world is unique and alive, and the continuity is maintained with extreme precision. The smallest scroll contains a scrap of information that jives with everything else PERFECTLY. Even if the information is useless to the player.

5. The main character. Garrett is a truly unique person. Cynical and jaded, he still possesses a sarcastic streak a mile wide, resulting in some priceless dialogue.
   
   The Bad
   1. Lack of rag-doll physics means that dead bodies get thrown into weird positions, which kinda ruins the verisimilitude (oh, look it up!) . There was nothing to be done about this at the time, but it'd be nice if the source code to the game was released so the fan community could improve on the engine. It worked for Doom!

6. Difficult to work with custom content. To make new missions, one must replace the existing missions. The game simply does not support loading custom content the way other games do (like Doom, for example). This doesn't affect the main game itself, so it's hard to really hold this against it.

7. Uh, I can't come up with anything else to complain about, but these lists always look best with at least three entries.
   
   The Bottom Line
   A steampunk cloak-and-dagger adventure, where a sharp mind is more important than a sharp sword.