I think this is the perfect sequel, it takes every single element of its predecessor and ramps it up a notch. It's such a natural progression, down to the architecture and technology, and takes further strides with its mechanics to challenge the player in ways that feel totally fresh and fair. It's all the best parts of Thief, but more!
The immersive sim elements are taken a step further, and missions will provide more unique player-driven goals than the previous game's more linear "get object and leave" objectives. Within the first few missions in T2 we get an "escort" mission that plays surprisingly unintrusive, a "grab as much money as you can" sandbox with optional secret bonuses, and a heist that challenges you to ghost as much as possible, with penalties to knock-outs and kills. The level designs are better, the tech is interesting and is actively shown being integrated into the world, and the puzzles have a lot more variety.
Every level has a gimmick to some extent, and up until the last few levels every single one felt like "the" level. There are only a few shortcomings in my book -- Garrett's new eye is mentioned once by Karras and never becomes relevant mechanically or otherwise, and there's a point where the mechanist plot peaked at the lighthouse level then dropped pretty severely for me with the reveal that the "horror of the seas" wasn't actually a lovecraftian creature, and was just a submarine they built to go to the ancient city, which also didn't house a lovecraftian creature.