What I liked about Thief was the concept and the execution of it.
Looking Glass dedicated themselves to a relatively simplistic gameplay structure, but they did it with tremendous skill. The implementation of stealth, and the atmosphere of thievery in a fantasy/steampunk environment was handled in such a way that you couldn't help but be immersed.
That said, I always felt the games were too "pure" and simple for me - as I tend to prefer gameplay that evolves and expands as you progress. In Thief - you pretty much knew what the game had to offer in terms of mechanics and toys within the first few levels. Which meant that there were fewer things to look forward to - and though I loved the challenge of sneaking around and finding all the loot - there wasn't much to gain from it. As in, I'd have liked a more expanded structure - with character leveling and more profound gear upgrades. More RPG - and I'd love to see them go further with what Thief 3 did, with a more freeform style and a larger city to roam free in.