I've only played as a mage so far, but I think the difference is just in which runes give more bonuses... for a warrior - strength, for a paladin - spirit, for a mage - magic...
Right now, towards the end of the game, I'm planning to max out all the skills for my mage, so he'll be a warrior and a paladin too, basically...
And also, I haven't played with the others yet, so I don't know... but most likely, they also give different bonuses when leveling up... for a warrior - attack/defense, for a mage - intelligence...

