More specifically:

Conclave improves internal politics. Powerful vassals get unhappy if they're not on your council, and your councilmembers get to vote on various items you do (war, revocation, etc), depending on how powerful the council is in your kingdom. They vote based on several 'personalities': Malcontent (will oppose the liege), Pragmatic (path of least resistance to strengthen realm and self), Zealot (religion at the forefront, holy wars and banish infidels), Loyalist (will support the liege), and Glory Hound (Go on! Take on the Karling blob! It'll be glorious!). It'll definitely change how you play. Used to be the only reasons to keep vassals happy was a) higher taxes, b) higher levies, and c), avoid rebellion. Now you need to keep certain interests aligned with yours as well, adding a whole new exciting level to the politics!