You're casual for complaining about a game that's aged extremely well. Thief's not without its faults, but the atmosphere and stealth mechanics are unsurpassed to this day. You sound like you're too used to zipping from cover to cover using cinematic takedowns.
So what are you, MLG Pro Hardcore with your call of duties? If it makes you feel any better, I have been playing it more, and it has been growing on me. Now that I've kinda got a feel for the movement and how the guards react to certain speeds it doesn't feel quite as slow paced, since I better understand how things work. But I don't think it's totally flawless. Better than a lot of modern stealth action games? Sure, I'll give you that, most modern stealth games are too easy, I love Splinter Cell Blacklist, but it's very easy to run through capping guys in the head. Sniper Elite 3, and Mark of the Ninja are very good examples of well done stealth action games, in my opinion.
Anyways, the issues I have are that the movement controls are a bit awkward, since it uses WASD and X, they tried to cram a lot of various movement speeds in there. It's not terrible or anything, but it's awkward, and takes getting used to, it could have been simplified a bit I think. One of the bigger gripes I have is that the guards can be pretty unpredictable about when they can and can't spot you as well. I've had them bumping right into me and not spot me, and then I've had them spot me immediately from a distance.
I still think that the game is too punishing when you get spotted, I understand it's a game about thievery and stealth, and that being caught should be a punishing experience, but it really just feels unfair, rather than giving you a hearty challenge to overcome. Your primary means of self defense basically amounts to you mashing left click on the guard until they fall over, which wouldn't be a bad thing if they didn't take forever to die, by the time you've killed a single guy, you probably have other guards on your ass. Your other option is to run and hide, but it feels like the guards can outrun you. I just end up reverting to last save since trying to hide or trying to defend yourself or evade the guards feels pretty much hopeless.
The last issue I have is more of a nitpick, but it kinda bugged me, in a couple levels you encounter zombies, and weird underground monster things, which really threw me off, because they act totally different from the guards I had grown used to. Neither of them were really difficult to overcome, zombies are slow as shit, and the monsters die easy, but it's kinda tedious, and throws off the games pacing.
You sound like you're too used to zipping from cover to cover using cinematic takedowns.
I don't really see what this has to do with anything. You still go into cover in Thief, to peek around covers, from behind things, etc. The only difference is you're not hitting a button to hug a wall like in Splinter Cell or what have you, but they both serve the exact same purpose. I get into cover in Thief to safely peek around corners, and I do the same thing in games with cover systems.
As for cinematic takedowns, again, I don't really see much of a difference in holding left click and clonking a guy on the back of the head in Thief to knock him out, or going up behind a guy in Dishonored and briefly holding the mouse button down to get a quick animation of me choking him out. It has the same desired effect, just one has a little more flair.