'Superior' armor sets are listed by class because its most appropriate to allow the appropriate class to get first roll on a item if its their actual class set. As much as a warrior may want a paladin's set item or a shadow priest wants the warlock set, do the right thing and allow the appropriate class first dibs. It can often cost you your reputation in end-game grouping if you get too greedy.
All classes (except Death Knights) have a Superior blue set, a Zul'Gurub set, a tier 1 and tier 2 Epic set. All tier 1 epic set items drop inside Molten Core as do the Tier 2 epic pants (from Ragnaros). Tier 2 helms drop from Onyxia, and all other tier 2 pieces drop inside Blackwing Lair.
Patch 1.6.0 (12-Jul-2005): Set items which are broken no longer contribute to set bonus listings in the item tooltips.