Not all fights require all (or even any) players to have resistance gear and/or protection potions, but here's a general list of bosses that have some sort of elemental damage component involved:
Arcane - Shazzrah, Prophet Skeram
Fire - Onyxia, Baron Geddon, Ragnaros, Vaelestraz, Firemaw, Four Horsemen
Frost - Sapphiron, Kel'Thuzad
Nature - Princess Huhuran, Viscidus, Maexxna, Grobbulus
Shadow - Doom Lord Kazzak, Loatheb, C'Thun trash mobs, Four Horsemen
