There are several kinds of raid bosses, and while from time to time blizzard makes something very original, most stick to some sort of template. There is a poll attached, and here are the explanations for each.
Multi-stage, w/ transitions - Lich King, Lei Shen - Different phases that have slightly different mechanics, seperated by transition phases that are themselves as difficult as fighting the boss.
Multi-phase bosses - Feng, Spirit Kings, Iron Qon - Fights that have distinct phases with completly different mechanics.
Councils - 3+ bosses that need to be handled, usually contains many mechanics to deal with all at once.
Dragons - Frontal breath, various avoidance mechanics and adds to deal with.
DPS races - Garajal, patchwerk, gruul - Must dps quickly before boss enrages, few difficult mechanics, but must be on your game to defeat.
Twins - 2 bosses that interact somehow with each other. Twin consorts, captain planet fight in BoT (even though there are 4, only fight 2 at a time), the sisters in Sunwell.
What is your preference?