Loatheb used to be pretty ridiculous. Everyone in the raid needed to have shadow protection potions, bandages, healthstones, the things from felwood and needed to be totally stacked with consumables (this was back when you could have multiple elixirs and flasks and food buffs all at the same time). In addition to the huge consumable farming requirements, you would also commonly see guilds collecting world buffs (Onyxia and Nefarion head, Hakkar’s heart, Dire Maul North buffs, etc.) to get their first kill. The current version is not a pushover by any means, but he is nowhere near as difficult as he used to be.
The major challenge in the fight comes from the healer side of things and it is a pretty unique challenge. The boss periodically puts a debuff on the raid called Impending Doom that does a bunch of shadow damage when it runs its course. It can’t be dispelled or resisted, you just have to eat the damage. But the twist is that your healers can’t just heal up the raid damage like they would in any other fight because Loatheb has an aura that is active 17 out of every 20 seconds that reduces all healing effects by 100%. The level 60 version was slightly different in that if you cast a healing spell, all of your healing spells would go onto a 1 minute cooldown. With the number aoe heals available to the raid these days, its not terribly hard to get the raid healed up after an impending doom so its not necessary to have the raid stocked with consumables to heal themselves and/or absorb the damage. The other part of the fight is the spores. From the 4 corners of the room, spores spawn and they float toward Loatheb kinda like the power sparks in the Malygos encounter – when you kill a spore, the 5 closest gain a “debuff” called Fungal Creep that increases your crit by 50% and reduces threat generated by 100% and lasts 90 seconds (I think). You will want to have the groups in your raid cycle through so that everyone gets the debuff.
The fight used to be a huge DPS race. Healing classes didn’t have nearly as many AoE heals and since each healer could only use one healing spell per minute, it usually went to the tank so the fight was all about killing the boss before he killed the raid with Impending Doom damage when bandages, healthstones, and shadow protection potions were all on cooldown. Since you had 40 people, you needed a strict rotation to pick up the spore buffs so that everyone would have it as much as possible and even the tank would want to pick it up once he established a threat lead for a little bit more damage. Now, its nowhere near as strenuous. As the tank, you can pick up the debuff if you want, just keep an eye on Omen because hunter pets might creep up on you while your threat generation is zero.
Healing can still be tough if your healers aren’t paying attention and you might have a few people die along the way, but its not a huge problem because afaik there isn’t an enrage timer so the boss will die eventually (I think there is a video on youtube of a prot warrior and a holy pally 2-manning the encounter over the course of 3 hours). One thing I know our resto druids like to do on this fight is to put up a Regrowth on as many people as possible, even though it does zero healing while the necrotic aura is up, so that when the aura goes down they can hit wild growth and get a huge burst of aoe healing during the 3 second window. The other healers will time their casts so that the moment the aura goes down, their cast finishes so they can squeeze 2 or 3 heals into that 3 second window.