Thief 1 had kind of a messy development, and it showed in the final product. They didn't know quite what they wanted it to be until late in development, and as a result there are some really terrible missions to go along with the really good ones. Thief 2 is the better, more consistent stealth game and ultimately my favorite of the series, but Thief 1 is still a landmark game and it's tremendous when it's at its best.