What I meant before is that there aren't classes in Divine Divinity, but you can have a character that represents one or more known classes.
For class related quests, perhaps the quest would not be provided until they feel that you are worthy of it (by checking your data-file <img src="/ubbthreads/images/graemlins/rolleyes.gif" alt="" />). You cannot join the fighters guild until you have level 3 in any one Weapon Mastery, Strength of 50, and Agility of 30. As for the healers guild, you'd need level 4 Restoration, level 2 Bless, and 60 Intelligence. These requirements would never be told to the player.
The only problem might be players creating a super-multi-classed character just to get into all of the guilds.
What about one guild membership only for the entire game?