Thief II: The Metal Age is technically a sequel -- it's in the name, after all -- but it's so much like its predecessor, Thief: The Dark Project, that it doesn't really feel like a sequel. It's basically the same game, but with 15 new missions, a couple new items, and a few technical upgrades to the engine. I guess Looking Glass Studios realized they had a pretty good formula on their hands, and chose not to do anything too extravagant with the sequel. The two games are so fundamentally similar that people tend to lump them together as one collective entity, because if you like one, then you'll like other.
And yet, people definitely have their preferences, with some people liking Thief 1 more for its darker supernatural atmosphere, and others liking Thief 2 more for its more robust level design. Some people find the undead enemies in Thief 1 to be a turn off, while others think the same of the robotic enemies in Thief 2. Having played Thief 2 immediately after finishing Thief 1, there are certain things I like and dislike about it; it's a tough call trying to pick one over the other.
I'm not going to treat this article as a stand-alone review of Thief 2, because that would feel mostly redundant, since I've already covered all of the basics in my review of Thief: The Dark Project. Most of what I wrote in that article applies to Thief 2 as well, so I'd recommend you start there so you have an idea of my thoughts going into this review. I'm also not going to make this a direct side-by-side, in-depth comparison article, either, because the games are so similar I can't talk at much length about how they differ. Rather, this review will be a more simple look at Thief 2 and, in general, how it stacks up to its predecessor.