Overcrowding will likely be based off of the first (and possibly third) example you said. I'd imagine an NPC would think that a 4 block wide 10 block tall room is rather crowded, and it's possible that NPCs having rooms all adjacent to eachother could count against that too. They can't just give away every single detail of the update or else it won't be as fun to discover when we start playing.
But... it's not going to be "fun to discover when we start playing". Or at least, it's not going to be for some people.
Maybe you enjoy building a house, then checking a stat to see if it changed, then rebuilding it again if it didn't change. Maybe you enjoy doing experiments to find exactly how close is too close or how small is too small or whatever.
I do not.
But even if you do enjoy this kind of experimentation, it'll take maybe 30 minutes before you know exactly what the overcrowding rules are. And that knowledge will transfer over to each time you make a new map. So the "fun" here is quite limited even for people who enjoy it.
Me personally? I'll just read about it on the Terraria Wiki; I wasn't planning on starting to play 1.4 for a day or two anyway, just in case of an emergency patch due to a crippling bug (especially if it affects world conversion).
Personally, I enjoy digging underground and finding caches of minerals and loot. I enjoy exploring the vastness of the world, seeing its breadth. I enjoy carving out an area for my home and personal use, a place where everything is exactly where it ought to be to make my life in the game the easiest it can be. I enjoy exploration and personal expression.
This feature does not enhance any of the parts of the game I find enjoyable; it makes the parts of the game I find less enjoyable more prominent, but not in a way that's actually more interesting. It's just more busywork: Build a bigger, more empty base. Build mini-bases in other biomes. Build one-off houses in the nearest X biome so you can get a fast-travel Pylon. Etc.
This feature is not terrible; it's not going to ruin the game. It is merely an
irritant, a feature that is
needlessly annoying in some way. And quite frankly, Terraria has plenty of irritants; the designers didn't need to add more. That is what annoys me the most about it; there's no reason they couldn't have added fast travel
without binding it to this irritant.
When they said they're introducing a new Golfer NPC, you didn't see people complaining since they didn't also tell us the specifics of all the items that they sell. They got excited because "wow there's a new NPC that will be related to golfing!!" just like how this post intended for us to think "wow there's a new dynamic NPC mechanic that has a lot of depth to it!!"
They don't have to tell us every single detail of the new mechanic in their spoiler.
The problem with your analogy is that you don't seem to understand the ramifications of the feature. If the Golfer NPC is worthless, if he doesn't sell anything interesting or if the golfing mini-game is as dull as fishing... so be it. It's not like the game is lacking in NPCs of limited value. If you want to ignore him because he doesn't provide anything you want, it amounts to nothing more than one additional box to build.
This feature is different because it affects
our home, the place we build and curate over the course of the game. The safe place, the place where we should feel most comfortable.
The exact specifications of this mechanic will decide exactly how it will change the act of building our home. If someone came to your house with an excavator and said that they're going to dig up some stuff, don't you think that sounds a little... vague? Is it unreasonable to ask them to be a bit more specific about their treatment of the place you
live?