In The Burning Crusade, the 5-mans had trash galore. Five, even six packs in Shadow Labyrinth, constant streams of adds in Shattered Halls, complicated trash pulls in Karazhan and Serpentshrine. I still remember with dread being the tank standing there using spell reflection to get the attention of Hyjal caster waves. Dealing with trash has always been a part of the game, as it has been part of the genre. In fact, one of the reasons that Naxxramas 10/25 felt so empty to me was that it simply held less trash than it did as a 40-man raid at level 60, so huge stretches of the place were deserted, as if Kel'Thuzad had gone on a staff-cutting binge and pink-slipped half of the Scourge before you even got there.
After we'd gotten Ulduar more or less on farm and moved on to Trial of the Crusader, I was pretty ready for a break from trash. I was very excited about a trashless dungeon where you just fought bosses. Wouldn't that be epic? Wouldn't it be awesome for every fight in a raid to be an epic struggle against a named, powerful adversary?
Turned out, not so much.
Removing the trash from TotC didn't make it feel epic. Partially, this could be due to the other design change of TotC, namely that it takes place in a single round room with no real distinguishing features. Combined, these two facts make the bosses actually start to feel like trash pulls. I did Faction Champions my first time in a kind of bored daze, wondering why it was taking so long to kill them. Ironically, instances like Black Morass and Violet Hold, despite basically being the same (big area, you don't go anywhere) managed to be more compelling than TotC because they used variations in the terrain and trash spawning in waves to break up the tedium of the big round room. Perhaps TotC would have been better with a few naval battles just like actual coliseums had.
There's a fine line in trash difficulty and, more importantly, trash respawn times. Trash difficulty should be high enough to engage your group and get them focused, and this difficulty can often be used by designers to give you a sense of what the boss mechanics are going to be. This is a useful role for trash, to get players looking forward to the boss and what it may or may not do to them. If the trash respawn is too long, once you've cleared it, then it may as well not matter at all. If it's too short, it ends up chewing up limited time you may have to learn a boss's full suite of mechanics and work on defeating him.
Take two bosses, Halfus Wyrmbreaker and Magmaw, and consider their trash and timers. Magmaw has three trash mobs, one with a straightforward suite of abilities and two with a charge/switch mechanic. These trash pulls don't take long to deal with once you know the mechanics, but they don't particularly educate you about the boss and serve no real purpose. However, they're at least easy to deal with, so their two-hour respawn time is not onerous. (Although I'm not discussing them here, the trash for the Omnotron Defense System is superior in that it is educational in nature.)
The trash pulls on the way to Halfus Wyrmbreaker, on the other hand, are excruciatingly tedious. They involve mechanics based on other sections of trash (the glowing orbs that do massive damage to anyone) that require a group to learn on the fly how to deal with them. They teach you nothing at all about the Halfus fight. And there are a lot of these pulls. In this situation, there is absolutely no reason for a two-hour respawn time for this trash. Forcing people to reclear this trash every two hours when first learning the boss is, in effect, working in a "We don't want you to spend your three hours a night working on this boss; if you can't get him in two, we want you to call it" mode -- which is fine, if that's what you're going for. But it's a punitive design choice, one that punishes you for being new to an encounter.
Getting a boss to 20% over the course of a night, being near the end of your raid and wanting to make that one more attempt that you think can push you to victory is a fun part of raiding. Being unable to do so because all the trash just respawned and you only have 20 minutes left in your raid is not a fun part of raiding, and I don't think it's a good design choice to force time-outs on us. The two-hour respawn, in this case, basically penalizes your raid for taking a raid break, for having a tank AFK because her husband called from work or what have you.
Honestly, the best trash I've seen in the past three years of raiding was Ulduar trash. It often taught you what to expect in upcoming fights, it was numerous and complex enough to be entertaining without being so numerous that it felt like a slog, and it engaged players in how to overcome it without becoming more daunting than the bosses. So far, to my mind, Cataclysm trash is somewhat above vanilla and BC but below the bar set by Ulduar. You definitely need trash, but right now, we maybe need a bit of a cleaning service, since trash is actually serving as an artificial and pointless gatekeeper to content.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from leveling up a new goblin or worgen to breaking news and strategies on endgame play.