I have gone for world domination once (and only just managed it for a turn) and it it not all its cracked up to be- days of crushing poor forces and wiping out rebbellions for a single moment of acheivement. I rarely get as far as fifty regions now as by the time you have thirty- fourty 'good' ones the challenge has gone- I have saves of most factions now waiting for the day when I want to finish them off. I never do.