Leotheras the Blind in Serpentshrine Cavern is almost a perfect fight. Leo alternates between human and demon form, staying in each form for about a minute. As a human, he dual-wields and occasionally whirlwinds around the room, spinning towards random people. Leotheras wipes threat after the whirlwind and when he changes form.
In demon form, Leotheras casts an AoE Chaos Blast targeted on the person tanking him. Most guilds use a warlock in Fire Resist gear to tank him in this stage. Leotheras also summons Inner Demons for up to five members in the raid. You are the only person who can kill your Inner Demon, and if you fail to kill your Inner Demon in 30 seconds, you are Mind-Controlled for the rest of the fight.
This fight is--with one huge exception--a great deal of fun. Threat is wiped often, so DPS has a very stop and start aspect to it. It reminds me of the children's game Red Light, Green Light. As well, killing your Inner Demon is a thrill, especially for a healer. It's great fun to wear a bit of DPS gear, and be able to drop everything, and go medieval on some demon.
The big exception is Chaos Blast.
First, a bit of background on resists. There are two types of resists in WoW: partial resists; and binary resists. If a spell is pure damage, it is subject to partial resists. Basically, with partial resists your resistance works like armor, and prevents some of the damage on every cast. If you have 75% resistance (365 Resist) 75% of the damage from each cast is prevented.
However, if a spell has a non-damage component (like a Frostbolt's slowing effect), it is subject to binary resists. Here, resistance works more like Dodge, and you have a chance of either avoiding the attack completely, or taking full damage. If you have 75% resistance (365 Resist) you take no damage 75% of the time, and full damage 25% of the time.
Chaos Blast has a non-damage component, and thus is subject to binary resists. Specifically, each time you are hit with a Chaos Blast, you get a debuff which increases the amount of Fire damage you take.
So what happens is that the warlock takes no damage when she resists a Chaos Blast, but takes full damage when she fails to resist. Therefore damage is very spiky, and the spikes get larger and larger as debuffs accumulate. The big problem is that late in demon phase, if the warlock tank misses two resists in a row, she can take over 15K damage in slightly over 2s.
This is extremely hard to heal, especially as the warlock is taking zero damage 75% of the time. Cast-cancelling heals helps, but it's very common for the warlock to die. Guilds usually soulstone the warlock, and have druids ready to battle res.
By my calculations, during a demon phase there's about a 15% chance the warlock will take a double blast when she has 6 debuffs already on her. That's pretty much a guaranteed dead warlock for guilds at that level of content. That means across the entire fight, there's a 56% chance the warlock will be killed at least once during a demon phase.
This is terrible design for a raid fight. And quite frankly, it ruins what is pretty close to a perfect--and above all, fun--fight. This fight would be so much better if Chaos Blasts used partial resists instead of binary resists. I'm not really sure how the debuff would be applied, but I'm sure that a solution could be found.
In general, binary resists are a bad idea for damage-dealing spells. They make fights way too swingy. It would be much better if the damage component was separate from the debuff component. The damage component should always be handled by partial resists, and extra debuff components can be handled with binary resists (because a partial debuff often doesn't make sense, you either get the debuff or you don't).