Reagents are something most people never think about until they run out of them. You fill up while you're in town, and everything goes along smoothly until you, say, jump off a cliff and ... OH GOD where's the Light Feather?
But most classes depend on some reagent or another for core spells (sorry, I'm not counting Levitate and Water Walking as core class mechanics.) Warlocks use Soul Shards for practically everything, although those aren't traditional buyable reagents. Mages need them to teleport themselves around and portal their parties out of instances. Shamans need Ankhs to self-resurrect, and druids need various reagents for their combat rez. Mages, priests, paladins and druids all need reagents for their full-party buffs.
Although rogues like myself aren't exactly known for buff spells -- the primary use of reagents -- we need Flash Powder to vanish, Blinding Powder to blind, and about seventy million different things for poisons. I'm surprised more rogues don't have carpal tunnel syndrome from buying hundreds of Deathweed, Maiden's Anguish and Essence of Agony whenever they're in town.
The kings of reagent use seem to be paladins and priests, who have to use symbols and candles (respectively) for most of their oft-requested buff spells. The lucky pallies, however, have their Symbols of Kings in stacks of 100, so they don't often run out in the middle of an instance. Hunters and warriors have no reagents at all, but it's merely a technicality for hunters, who depend on large amounts of arrows/bullets and food just to kill anything. And while warriors have zero cost for reagents, which makes some classes jealous, tank repair bills are not exactly low.
How do you like your class's reagent costs? Do you wish your reagents would stack higher or cost less? When have you found yourself reagentless at a bad moment?