I work as a game designer (nothing remotely related to MMO's) and I rave about Naxx to my guildmates pretty often. Nearly every fight evinces a unique design idea. Grobbulus is great, by the way.
Loatheb. (Actually the same designer did Grobbulus too, but I actually like Grobbulus -- it's a fun fight and I like the concept. It's just too easy.) I just can't wrap my head around how the same person produces the ultimate "skill > consumables" fight and then turns around and makes a "deposit gold for epics" moneysink. :(
Edit: (BTW, to ward off any conspiracy theories, my knowledge about who
designed what, etc., comes from interaction with devs on the Naxx PTR.)
So what does Loatheb have? Start with Corrupted Mind. Awesome idea. Next, you have to prevent the raid from stacking healers to overcome a raw numerical requirement. So make it a DPS race. That's nothing new--it's Patchwerk.
Now you can almost see where this is going. You have a fight where the DPS can't get heals because of already existing mechanics, and you want them to die after a fixed amount of time. It tidies up an encounter idea with some neat design (Fungal Bloom isn't really relevent--you could have thrown that into any fight that needed to have a little more "going on").
But then what happens? It's not this guy's fault that consumables exist in the game. It's true, though, that there's little he could have done about it. Maybe he realized at some point that Corrupted Mind wasn't such an awesome idea after all, because in the context of the rest of WoW (which has consumables), it runs into a conceptual problem. By reducing spell-based healing to a miniscule amount, the aggregate effect of consumables, which have remained constant, becomes overwhelmingly important.
So what does he do, when there's maybe a week before he has to have his content locked down for the PTR, and he has this fight to deal with. You obviously can't balance it for a raid without potions and just leave it at that. Basically:
1) Have Loatheb hang a "no potions" sign on his door.
2) Scrap the Doom idea and give him a vicious Berserk.
1 is horribly unpalatable. There's no way to hide that in flavor, and it's not even great for design. It ruins a certain feeling that consumables have always had, as relible aid for players. Once they break a barrier by taking them away from us for a certain fight, they've lost something.
2 just leaves you with a phenomenally boring fight. Expensive or not, the potions are at least something that the players have to do during Loatheb, a fight already lacking in real action of any kind. It would barely have been worth putting into the zone (especially with Patchwerk already existing).
What would you have done if this fell into your lap?
Remember a long time ago (maybe during BWL) when you said in a similar discussion that Blizz could never make a healer run out of mana? That's not literally what's going on at Loatheb, but I think the parallel is clear (and I'm out of time here, so I won't spell it out at the moment).
How the game has changed since then.