Fortunately, most of the really bad trash clears are a distant memory, but there was at least one recent one that almost everyone who raided Dragon Soul could agree on. I'm going to include both dungeons and raids here, mostly because Shattered Halls was among the first things to go on this list. After including that, I knew there were other, equally nightmarish 5-mans that had to be included in the interest of fairness.
For a bit of fun, the movie above is a Moviewatch entry from June 2007 showing a paladin at 60 killing most of Gnomeregan's trash in one pull. Notice how close he/she gets to death on multiple occasions despite the level advantage.
Molten Core I wasn't around for vanilla WoW, but I have yet to meet a classic player who has anything nice to say about the core hounds, lava packs, or the sheer depressing number of them. What was the thing about the core hounds? They all had to die within a few seconds of each other or the entire pack respawned?
The first time I saw the raid during BC, I turned to one of my guildies (an experienced classic raider) and said in amazement, "You guys seriously had to kill all of these mobs?"
Temple of Ahn'Qiraj (AQ40) The Twin Emps to C'thun trash has reached legendary status in the WoW community, and not without reason: It inadvertently caused one of the first high-profile events in the game. Raiders from the guild Overrated (Black Dragonflight-H) were permanently banned over an exploit that allowed them to skip trash and go straight to C'thun. While most players disagreed with their cheating, Overrated wasn't without sympathy, particularly from other AQ40 raiders who may not have approved of their choice, but agreed with the impulse behind it.
As RivanLord noted in the comments, the underlying problem was the rapidly-aging design sense behind the older raids. You can't ask 40 people to do 4-5 hours' worth of trash to reach the one boss they actually need and call it fun.
Heroic Mana-Tombs Personal anecdote: I first learned how to tank in Burning Crusade heroics, and heroic Mana-Tombs terrified me almost as much as the next two on this list. No matter how smoothly things went elsewhere, I almost always lost my healer or a clothy at one point thanks to the Nexus Stalkers and their Gouge ability. This was back when taunts were melee-range only and mobs instantly dropped aggro on an incapacitated tank. I'd get gouged off the pull, sit there stunned, and watch while the mobs two-shot the healer and then ran for the fleeing DPS.
And then there was the Ethereal Darkcaster that burned most of the healer's mana, forcing him to drink each pull, and the Crypt Raiders that charged and stunned, the Theurgists that decimated the melee DPS with Blast Wave, and the Spellbinders that silenced. Mana-Tombs was Satan's own crackerjack box, stuffed full of awful prizes, and each more evil than the last.
Then add the BC-era destro warlock and no Blessing of Salvation to the group, and someone who pipes up as you start the pull: "By the way, I'm on Trial of the Naaru: Mercy, so we need to move quickly."
That, right there, was every rage tank's definition of 5-man hell.
Heroic Shadow Labyrinth One of our commenters memorably described Shadow Lab as having "more trash than a New Jersey landfill," and oh, ye gods, was it ever ugly. Stunning, bleeding, poisoning, charging, fearing mobs coming at you like an enormous, starving wolf pack, and Cabal Shadow Priests with a 2.5K per second Mind Flay on tanks that usually started heroics with less than 10K health.
Take Mana-Tombs trash, bung an additional two or three mobs to each pull, and increase the size of the instance. That's Shadow Lab.
Karazhan -- Curator and Shade of Aran trash This was the longest stretch of trash in the raid and everyone dreaded having to do it. Giant robots with improbably large aggro radii + magic-immune (and early on, bleed-immune) mobs = the unhappy midpoint of every Kara raid, and the spot where people reliably vanished for long bathroom and smoke breaks.
Some mention should probably be made of the Skeletal Ushers on the way to the Opera event too, at least before they could be crowd-controlled.
It also spawned this gem of a comment from Endure, a player in Crimson (Greymane-A), and one of the sorry souls fighting through mountains of SSC trash:
Tempest Keep All of the problems of SSC trash with even more annoying mechanics! Anyone who off-tanked TK will probably have memories of sitting bored in a corner of the phoenix room waiting to get bum-rushed by the birds' frequent charges. Oh, and whoever resurrected Kael'thas for Magisters' Terrace also must've resurrected his trash, because heroic MgT was no bowl of chuckles either.
We want trash while we are fighting bosses. We want trash on shorter respawn timers. We should have to fight trash as soon as we zone in. No buffing time. No time to group up. We should have to fight trash when entering Shattrath, Ironforge, etc. We should have to fight several trash packs to get to the bank. Opening the bank should require a trash pack. Trading something with someone should spawn trash. Going to the mailbox: trash. Crafting an item: Trash. Gathering a resource: Trash. In fact, it should spawn several trash packs that all chain local mobs. Mounting: Trash. Logging onto the game should require you to fight trash to make sure you are worthy.
The Battle for Mount Hyjal 95% of this place was literally trash wave after trash wave. Yes, there was an actual lore reason for it, and yes, it was actually kind of fun the first or second time you did it, but it got old pretty quickly. A big nuisance for raid leaders was trying to make sure the raid was actually prepared for bosses that came at a scripted point after the last wave. I have memories of our raid's rogues working in tandem to Distract Azgalor long enough to give us enough time to resurrect, heal, drink, and buff.
With the benefit of hindsight, The Burning Crusade was a pretty awful experience trash-wise. It was like a festering, ever-increasing pile of endlessly respawning suck that oozed into every orifice of the playing experience and could only be avoided by giving up and logging off.
NB: The secret was not to use the little alcoves off to the side, which bunched the entire group into an easily-Flamestriked corner by the Phantom Mages. If you kept the group around the entrance hall, the healer and melee mobs could be killed as long as the tank kept some threat on the hunter and the mage. I have to admit that heroic HoR eventually grew on me as a fun challenge, but I couldn't blame the legion of players who'd had bad experiences there and wanted to bolt as soon as they saw the loading screen.
Heroic Stonecore I thought I'd seen the last of players' dropping instantly after zoning into a 5-man, but I was wrong. For the first few months of Cataclysm, heroic Stonecore was pure agony for PUGs, and it was a pain in the ass to tank through the end of the expansion. The opening hall had its Stonecore Earthshapers, all of whom had to be watched like a hawk before they got a Force of Earth cast off, and the Berserkers whose Spinning Slash made melee hell to keep healed. After that, it was just a simple matter of getting through the trash-infested hall to Ozruk with its multiple add-summoning casters and giant pathing ogres!
Heroic Vortex Pinnacle Did anyone else get the uncanny sense that Vortex Pinnacle was the result of an internal competition at Blizzard to see who could program the world's most uncooperative, CC-resistant, annoying trash packs?
- Classic Naxxramas: Interestingly, the damage done by the mobs in classic Naxx isn't that much less than the damage they did in the revamped Wrath version.
- Heroic Magisters' Terrace: Kael dies again, but not without having made you suffer for the privilege.
- Oculus: The vehicle component's come in for its own share of abuse, but the high-damage mobs on the central ring didn't help either.
- Baine trash in End Time: Rare was the group that didn't butt-pull at least one trash pack.