At the risk of angering the entire Enworld community, I'd love to see Multiclassing go the way of the Dodo.
Or, if Multiclassing MUST be endured, I'd like the following rules:
1) You can only have 2 classes. No Fighter 3/Rogue 3/Sorcerer 1/Druid 2.
2) At least one of the classes needs to be Cleric, Fighter, Rogue, or Wizard.
3) When you Multiclass, you must continue taking levels in the 2nd class until your 2 class levels are equal. After that, neither class can be more than 1 level off from the other class (optional)