That's exactly it. Pretty much every five meters you think "Whoever created this actually gave a shit about the whole product". It never feels like things are worse than they should be, or that they could have been better with a little effort.
It's the kind of game where everyone who worked on it can be very proud. Do you think the average Blizzard developer these days can say the same?
There's a lot of griping about the third act, and yeah, I get it, it's incomplete and comparatively poorly optimized. But it's still really good. :P They clearly bit off more than they could chew with the City, that maybe they could've solved with another year or two of dev time. (Hopefully, will be solved in a Def Ed a year or two down the line.) That isn't necessarily dev time they had, though. Not without taking out a ton of loans (do you really want tencent to own more than 30%?) and risking ill-will with consumers and WotC. (The game was meant to come out over a year ago.)
I don't know what your problem is with Act 3. It might not be as good as the rest, but it's still plenty good and better than most modern games. I just killed >!Gortash!< after completing nearly every side quest I could find and I haven't really had any issues. Sure, there's a few things that could be a little more fleshed out or that feel like cut content, but not much. I also don't know how many of those things are actually cut content and not just references. Like in Cazador's manor there's >!a body with a note talking about a sleep potion and acid poison. I'm pretty sure it's just a Romeo and Julie reference. He took the sleep potion to fake death, his lover found him apparently dead and killed herself. He found her dead and killed himself.!< There may have been plans for a quest there, but idk.