Well one obvious thing is that you get skills by using weapons and those skills are locked to the weapon (if you use the lightning sword you get a skill called thunderclap but that skill is locked to the lightning sword and cannot be used by other weapons)
Some classes can promote 2 times (Mercenary>Myrmidon>Dreadfighter, etc) and some only can promote once (though it was implied that there is going to be DLC classes).