To clarify exp penalties..
In Dungeons and Dragons, a 'multiclass' is possible, but at a penalty. The complete ruling is at the following link:
http://www.dandwiki.com/wiki/SRD:Multiclass_Characters
The brief overview:
Levels are based on exp gained. Once a character reaches ECL (effective character level), he may choose to increase his class level to 2, or multi class level (say rogue/wizard) to 1rogue/1wizard. He would gain experience at a level 2 character, but the only reason a rogue might take a class in wizard is to read scrolls and learn level 0 wizard spells. He will gain exp as a level 2 character in the case.
How this applies to EQ: Every MMoRPG is based off of D&D. Classes, races, etc... you get bonuses, and have negatives based on your race. Your class is pretty much your profession. Using the explanation above - instead of levelling each of your class levels individually, EQ has the following multiclasses: Paladin (Cleric/Warrior), Shadowknight (Necro/Warrior), Ranger (Druid/Warrior). It imposes an exp penalty to make up for levelling each class individually, and it's faster than the penalty of levelling two classes to the same level in D&D.