Thief 2 is more streamlined, accessible and comfortable game with better overarching story. It lacks raw, adventurous feeling of the first game.
I would say it just another spin on this whole "Thief" genre and it works. It is still a Thief game, with all the good parts that come with it.
Dark Engine is amazing, considering when the game originally came out. You feel like you can fully use environment to accomplish your tasks.
Levels are very good, very elaborate, despite the game mostly being one big heist. All areas are very distinctive with a lot of personality.
Once again saying this, if you wanna have the intended experience, you need to play on expert and do not use guides.
Overall, an outstanding experience.

Thief and Thief II are as important to the stealth genre, and gaming in general as Half Life or Halo are to the FPS genre. The atmosphere, design, art direction, sound, and timeless gameplay with all the options you have on every mission have never been topped in the genre. In fact subsequent games have usually been dumbed down, though Dishonored 1 was amazing and did come close to Thief/Thief II's greatness. I have been playing these games since the 90s and still adore them. Of course these days I play with the HD texture mod and Tfix patches applied to 1 and 2 and that just puts the icing on the cake.

Buy it, mod it, play it