I can't recommend this game enough, even with its age and quirks.

But I always disliked the additional missions of the Gold edition, they are very unpolished and sometimes outright ugly.
Which is why I was happy to find out that there's a mod called "GoldToDark" which converts Thief Gold back to the original. Of course this isn't 100% (e.g., there are still the additional fire-guys in some of the missions), but it works well enough. After patching the Steam version of Thief Gold with this, and then using TFix, I could play the game "as it was meant to be". (Windows 7)

Awesome!