A classic for its time. Simple as that. Set the precedent for other similar games (e.g. Assasin's Creed).
The levels were awesome--right from the start you really got a glimpse into the character and was immediately attached to him.
Overall, the gameplay was awesome. Sure the computer intelligence level was sometimes a bit obvious, but that was normal for its time. The dialog between the computers was sometimes very funny as well. It's just a great game that you'll love playing. I also enjoyed the second and third installments as well..If Eidos interactive didn't go out of business, I'm sure a fourth Thief would be on the horizon.
All in all, ergo, whatever you want to say...Thief: Deadly Shadows is a great game from start to finish. Go buy it, it is probably a $9.99 discount by now. What a bargain!