It's a reference to part of the deathknight lore:
Built by the skeletal architect Obrahiim and said to be so powerful that not even the dread citadel of Naxxramas would be able to withstand a direct assault If I am recalling correctly, there's an in-game reference that says the same thing or equivalent, maybe quest-text from Mograine? Can't recall that for sure, however.
You can check out more at wowwiki:
http://wowwiki.wikia.com/wiki/Acherus:_The_Ebon_Hold
I don't mind Acherus being our class hall, in fact other than the fact that it doesn't possess the "shiny and new" element of a brand new class hall, I like the idea of using Acherus. It does seem to fit really well with deathknight history and the fact that it's mobile makes tactical sense for the knights to employ it.
Having said that, I add my voice to the others here who have said: I
really hope they update some textures and maybe add some environmental doodads like more npc gargoyles, abominations, liches, crypt fiends etc. to the area. Give it some polish and add that "shiny and new" element that's missing right now.