The cult of Thief really is fucking insane. You can't say anything about the game is bad or they will sperg out instantly.
Just for the sake of it I will list some things I find flaws with Thief 1 (gold) and to a extent with some of these, 2.
Many levels are designed like garbage. Only mazelike in order to inflate game time or sell guides or whatever the fuck. Loot placements in many are also completely nonsensical, particularly The Haunted Cathedral comes to mind, which on the hardest difficulty you need to get around 3000 out of 3,5k IIRC and most of them are fucking tiny and in places that make zero sense, like a tiara on a pile of leaves which has no purpose at all. Another that comes to mind would be that mission where you get your lockpicks, where in order to finish the mission you have to find a fire poker which is hidden in a room that can only be accessed by pulling a lever inside a fireplace that you would only know about if you happened to look inside it (which there is no reason for, considering that this is the first time that something has been hidden in a fireplace, and is never written or mentioned anywhere.)
Although I will give credit where it is due, as I think the map design, placement and many other things improve considerably after The Haunted Cathedral. I think Thief 1 would be really great if parts of Cragsleft were redesigned, Assassins would have that nonsense fireplace poker mentioned in some text or NPC chatter and have The Haunted Cathedral and Mages Tower redesigned to something less shit. I think the only bad mission after Mages Tower is Return to the Cathedral, but even that was better than the first Cathedral.
AI is flat out retarded, and even when fully alerted they are barely more intelligent. The Dark Mod improved considerably on this, with AI being impossible to blackjack once they are on full alert (although this is a decision up to the mapper, it is toggled on by default in Darkradiant), which means legitimate and permanent consequence. Their sense of sight and sound is also garbage, which adds to the next point.
Blackjacking is way too easy and can be done from any side. You just lean forward and hit them, you can do it while running on basically any surface aside from marble.
Healing supplies are stupidly plentiful in many missions, but what do you need them for anyway when you can knock everyone out so easily?
That's basically it for Thief 1. Thief 2 was far, far better overall. The only positive aspect for Thief 1 over Thief 2 is atmosphere, but it is very unfortunate that atmosphere had to be wasted on such shit levels. Thief 1 was a good first attempt but they made some bad decisions. Sort of like Splinter Cell 1 having forced action sequences that really ruined a lot of the game and Chaos Theory being probably the best stealth game of all time.