The aesthetic of it matched what I thought it would look like. The red and orange coloration, the dead looking sky with gargantuan tentacles in the horizon, tall black spires and evil-looking architecture (I'm unsure what kind of cultural nod the Old God-like architecture goes to) and everything.
^Warcraft Chronicles depiction of what a typical day during the reign of the Black Empire looked like.
^I think they did very well with maintaining that look. Perhaps not as "dark and gloomy" as the Chronicles art but they can only do so much to represent that in-game with how the general art style in the game is.
In my mind if they expanded on Ny'alotha, making it more than just an instanced raid, we could be able to see legions and legions of Old God minions marching in the background.
The visual of physically seeing N'Zoth in the background during the Carapace of N'Zoth boss fight is fucking cool. I think it would even cooler if we Ny'alotha was an actual zone - or continent of zones - that you would always see N'Zoth in the background (or skybox) so no matter where you were questing you knew what was waiting for you.
It should have definitely been more than just a one-and-done raid. But I understand that the majority of players do not like the theme of Old Gods and their theme. That's understandable.