Thief Gold is a solid re-release of Thief: The Dark Project, but the new additions and changes don't seem to fit in.
Thief Gold plays exactly like The Dark Project because it is after all the same game. Your goal is to use the shadows to your advantage, and silently move through the game's many missions stealing loot and following the well told story. Thief Gold adds three new missions to the table: The Thieve's Guild, The Mage Towers, and Song of the Caverns. The Thieve's Guild is more or less a side story mission, and takes place before the actual plot of the game begins like the first four missions in the original game. The Mage Towers and Song of the Caverns are missions that hold two of the four talisman's you have to collect that were originally located in two of the original game's missions (which both held two talismans). The only problem with these new missions is their steep rise in difficulty and length which makes them feel disjointed from the rest of the game.
New players may feel overwhelmed by Thief Gold's exclusive missions which take over an hour to complete minimum compared to the original missions which could be completed in twenty or so minutes minimum (on the normal difficulty). The new missions are rather unique, as well. Unfortunately like the difficulty and length they feel out of place because of this. Thieve's Guild is pretty much a maze, The Mage Towers feels like it belongs in an RPG or Platformer, and Song of the Caverns seems more suited for Thief II than it does the original game. The new missions aren't the only content added in Thief Gold, either. Several changes have been made to the original mission's to help tie them in with the storyline better. The Haunted Cathedral now includes details on the latter two new missions, and the Lost City replaced the Craymen with Mages from the Mage Guild mission which in my opinion ruins the atmosphere. There is also an interesting secret area in The Sword. Then again, the entirety of that mission was interesting to begin with.
Everything else in Thief Gold is the exact same thing that it was in the original game. Hardcore fans will be motivated to play through the game again to see the new missions (though Gold comes with an executable that lets you force the game into starting at one of the three new missions), but new comers to the series may be overwhelmed by the steep rise in difficulty during Gold's exclusive and lengthy missions.