The Thief series gets even better when you start delving into the fan missions. In particular works like Gathering at the Inn, Inverted Manse and T2X: Shadows of the Metal Age
I've tried revisiting Thief 3 a couple of times and while there were changes in mechanics the one thing that jars me more than anything is the constricted feeling of the smaller levels and the frequent transitions. I can't help but wonder how the game would have played if it had not had those transition points that damaged the flow and sense of place of each level and location :\