When I play, I mostly play Empires. They just have cooler champions imho (a zombie champion > an elf champion in coolness). Recently I've started to play the Kingdoms a bit more, so if anyone has any ideas on how to make them cooler let me know.
Angels really should have a spell to turn shards into life shards.
Elves are bloody overpowered! Their drawbacks don't really make up for the huge mana income.
Centaurs badly need something to make them stand out as a faction. They have +20 Carrying Capacity and +1 Moves, but they need some spice.
Gnomes, Living Stone and Shadows all have their fun mechanics that separate them from the other factions.