In mount and blade each faction's guard either shot better or had better melee stats, and some factions had irregular units such as partisans and militia, right now since the class traits aren't in however, so that won't matter for a while.
So far from the dev blogs there are a couple differences planned, French Light infantry and Guards will get saber briquettes as a loadout option, and one faction already has a 3rd musician, which is the U.K with the bagpipers, perhaps France could get Cornets, etc.
Chosen Men [British] - The prestigious 95th. You're a chosen man! It makes you feel special.http://www.holdfastgame.com/News/Post?p=1024
As for the next faction, it will like always be decided by a Poll and so far it looks like either Russia or the Ottomans will win the next one.