The April Fools' joke from the Death & Taxes guild was a fake map of Black Temple showing a huge rat maze with a handful of bosses, thousands of trash mobs and a remark saying that the respawn timer on the trash was 20 minutes. Whether it is for loot or the challenge, people go raiding to fight boss mobs. Spending hours to clear trash mobs, only to see them respawn and having to clear them again, is highly annoying. With the trash respawn timers being generally short in the Burning Crusade raid instances, complaints have been piling up.
Fortunately these complaints have come from the top raiding guilds, which are the only customers Blizzard is listening to. So WoWInsider now has the story that Tigole announced raid changes for patch 2.1.0 which would make trash mobs easier to clear and respawn slower. (Unspecified changes to how Elixirs and Flasks work have also been announced. As have changes that make rogues better in raids.) And no, this isn't another fake joke.
I think a certain amount of trash mobs in a dungeon is fine. But I do agree that they shouldn't respawn so fast. The respawn timer in the first part of Karazhan is only 30 minutes, so if your raid isn't working like a well-oiled machine, and there is some delay due to somebody getting disconnected or having to leave, you get caught in a wave of respawning trash. I also wouldn't mind them thinning out the trash a bit more, as fighting the same mob over and over gets old pretty fast.
What I'm a bit dubious about is the design philosophy to make raid dungeons deliberately too hard to start with, and then gradually nerfing them down to an acceptable level. Sure, that is more popular than starting too easy and making it harder later. But a bit more playtesting would have been nice.