Whether or not Rares or Uniques are best in slot items is a matter of designers taste when it comes to Action RPGs. You can tune the item drop tables however you want. I would argue its nicer to have rares be BiS items, as its more fun when you see item diversity in the top builds. Seeing everyone walk around with the same Uniques is boring.

Diablo 3 fails because it does not provide enough ways to customize a character's build. There are a handful of stats that are important, and they mostly scale linearly. The skills are all unlocked, and you can change them at will.

If you do not have enough ways to customize your character, the item drop tables are moot. You can't drop items that are interesting if there are no interesting stats.

Path of Exile is the true successor to Diablo 2. The options for customization are endless, so every item that drops has the potential to be interesting. You should check that out if you are interested in ARPGs, especially something like Diablo 2. And if you are really interested in game analysis, their systems are really unmatched in today's game marketplace.

Diablo 3 is not a game in the Diablo genre. D3 is closer to something like Baldur's Gate: Dark Alliance, or the X-Men Legends games.

Edit: I focused on the itemization part of your post, but I totally missed the map part of your post.

I'm not sure map randomness was relevant at all in Diablo 2. That game was pretty much all Meph runs, and then Baal runs. The maps didn't really matter, you just teleported to the boss and hoped for good drops.