This is only partially true, I remember times in TBC where we would be working on M'uru or Kael'thas and have trash respawn ontop of us, there was a lot of trash in TBC, Sunwell had a lot, Hyjal had waves and if you wiped on a wave you had to start ALL over again, how could you forget that? BT had some awful trash, and I think SSC had the worst trash in the entire game, the list goes on, TK, Mag, Gruul, even Karazhan had a ton of trash. If you added up how much time it took to clear to all of the bosses, yeah you'd be dumbfounded. My TBC guild raided something like 5 days a week for TK, SSC, BT, Hyjal. It's not like Wotlk where you had TOC which had no trash, Ulduar which has very little trash between bosses, Sarth which had minibosses and small trash packs, Malygos which had no trash, I think the most trash so far has to be ICC, could be wrong.