I didn't care much for Thief3, and honestly, I thought it could have been a lot better.
I felt that the level design in Thief3 was fairly bland in comparison to the levels in both Thief 1 & 2. It almost felt like there was a predictability in each of the maps with their layout, and I thought that the size and scale of the maps were a fraction of what they were in the first two. After you play maps like Down in the Bonehoard, Break from Cragscleft Prison, Framed, Life of the Party, and *the best Thief level of all time* Return to the Cathedral, all of which are gigantic in size, Thief 3 maps just felt like meh.
I also didn't like how Thief 3 took the "no kill" rule out of Expert difficulty. To me, that was like the entire reason to play on Expert difficulty, as it added an extra parameter you had to follow. In both Thief 1&2, Hard and Expert difficulties also added on extra parts of each map with different missions you had to accomplish. Take "Down in the Bonehoard" for example: In Normal mode, various parts of the level were cut off. In Hard and Expert, previous doors that were locked were now open. This added for a lot of extra replay value in each map. In Thief 3, Expert difficulty just meant you had to fine 90% or more of the loot in each map, and I would spend a good hour just running back and forth after I had disposed of all the baddies looking for that tini-weenie piece of loot I had overlooked. I found it quite frustrating.
Garrett just moved weird in Thief 3 as well. It felt very consolish. In both Thief 1&2, Garrett had no body. Your mouse/keyboard just controlled the camera, and you could see your hands, but your actual body was non-existent. In Thief 3, Garrett now had a body, which is cool. But since you turned the head, and the body followed, I thought it created for very disjointed control of Garrett.
The dagger was also a terrible addition to the Thief series. Let's put it bluntly: I like playing assassin, but I'm not stupid. Both the dagger and the blackjack shared the same exact function. Only one made noise, and the other did not. The dagger also made a mess. This I thought was a terrible move, as the sword at least had a separate function over the blackjack. At least with the sword (T1&2), you could defend yourself in a sticky situation. In Thief3, you would pretty much left to running around trying to get away, and fairly defenseless if caught in a corner.
So ya, Thief 3 just sucked. This was a major let down, because both Thief 1&2 were my absolute favorite games of all time. I was a real Thief fanatic there for a while, but c`est la vie.
For those who haven't played it yet and love the Thief series, check out Thief 2 : Shadows of the Metal Age. This was a full length fan created campaign (free) for Thief 2: The Dark Project.
It. Freaking. Ruled.
Go play it!