As for the open world thing, it is that too partially. I mean it's not like "load next mission" kinda thing like earlier thiefs or many other games, even if at the same time it isn't the kinda open world than fe. AC IV. Then again most ACs don't have any meaningful indoors areas in the open world either to my knowledge, but I do agree that they should've been able to make the whole "free roam city" into one area without loading zones. The sloppy developing can be seen elsewhere too, like performance and the buggyness you mention, but none of these are really putting me off from the game too much, cos I think the basic fundamentals are still there and at least addictive to me and that is stealing things from everywhere and everyone!
The game can be made into quite a lot of things really depending how you want to play. Like difficulty and what all you leave on (I also turned most of the "helpful UI stuff" off for challenge) and what is your stance on lethal force etc. I for one refuse to kill any humans (but I have killed a few dogs to shut em up) and the difficulty level I have also dictates that knocking out any civilians is not allowed. Someone who is less interested in the whole "thief ethics" thing could just shoot a guard dead to solve many puzzles much easier or be more prone for combat against them, but I try to go as stealthy as possible and harm as few people as I barely can, cos being a thief isn't about killing or violence. Just like in Elder Scrolls games the Thieves' guild and Dark Brotherhood questlines are nothing alike.
I've played the game about 17h now and I'm just past chapter 5. I believe there are a total of 8 chapters. After the first chapter which was largely a tutorial in disguise I spent almost 4 hours just exploring the "open world" and stealing stuff there and solving some puzzles I ran into before I moved into chapter 2. There should still be plenty of sidemissions for me to do, but I do feel that during the early hours into the game I did quite a few of them of what are available in the game.
But yea, still really enjoying the game. So far my biggest problem about it was the non-human things in one of the chapters (sort of a spoiler, but it was in the trailers too). I just hope I wont run into those anymore for the rest of the game. One of things I've loved about Thief (like all of em) is how Garrett is just a normal human (even if with a mechanical eye
) and there's no overly silly stuff about him, which is why the lore going supernatural has always sorta bothered me, even if I absolutely loved all the "down to earth" gameplay.