Matewis: Thief 1 after Thief 2 will be a bit tough I think. I tried a playthrough of Thief 1 but the supernatural levels really put me. That and, to me at least, some truly confusing level design employed in several massive levels.
Yeah, I finished Thief 1 and 2 one or two years ago and I also felt the first game had some confusing and bad level design. Some of the levels didn't really feel like real places or buildings, some of the designs didn't make much sense, oddly placed bridges or corridors etc. I felt Thief 2 was better in that respect.
Also there were certain enemy types in Thief 1 that made the gameplay tedious on many levels. Zombies were one of them but there were a couple others too (I think they were supernatural too, like some ghosts). The only way to get rid of them was to use some holy water arrows or flash bombs or other objects that were pretty scarce overall, and sometimes just running or hiding from them wasn't a good option either as there were too many of them around.
I recall especially one level (Cathedral?) where I was standing on top of some stone block and below me were several zombies and those ghost soldiers moaning and trying to get me as they had detected me, and I really didn't know how to proceed. I didn't have any explosive or holy water arrows and I was running out of flash bombs so I couldn't get rid of them all either. Maybe I just outran them after several retries... With humans it was better, you could always try to knock them out from behind.
Thief 2 had less of such irritating enemies, IIRC there was only one zombie in the whole Thief 2. The only type of enemy I didn't like in Thief 2 were the helmet-wearing knights which couldn't be knocked out, so the only option was to avoid them altogether. I didn't like such forced sneaking avoidance, I rather tried to clear the levels so that I could search for loot in peace.
However, what I did like a lot in Thief 1 was its story and atmosphere. Thief 2 was also quite good in that respect, but not quite as good as the first one.