Yes the ending was too easy. The city sections in general were (to me) the weakest points of the game. Not that they were unfun, but I would have greatly preferred a few additional "proper" missions to the somewhat repetitive and quirky AI filled city sections.
Still, I thought Thief 3 was great, and a fitting end to the series. Just thinking about it makes me sad. The Thief series are the only games I have played as an adult that really moved me and excited me like games did when I was a kid. I still love tons of other games and remain an avid gamer, but Thief was really something special.
For any around here that have only played Thief DS, for the X-box, I cannot recommend highly enough that you get the first two games for the PC. Just amazing stuff. The Thief 2 mission, Life of the Party, is just amazing, and something I will never tire of experiencing!!!!