WHY can't we just do Multi-Classing the 'old-way' - that is, if you belong to two Classes - you have to earn Experience for each.
The Character then gains the abilities of both Classes, and takes the Average of Hit Points, the best resistances from each etc.
Furthermore, they are restricted to the lesser Armour available - but may take any/all the Weapons available to each Class. If the levels between the Classes become too 'uneven' (i.e. more than one level difference) the Character pays for it with a 10% penalty on all Experience earned.
BALANCE I hear people screaming from the highest tower to the deepest Dungeon - Its no secret that I believe vehemently that game balance comes from the Dungeon Master rather than a set of artificial constraints in a rules-set (White Wolf were the first to instigate this in their Storytelling games, and it became very popular throughout the RPG Industry) - Rules Constraints do indeed make RPG's easier to run for newbies and more accessable to the masses - but IMHO they can also bland out the flavour and suck some of the fun out out the whole experience for both the DM and his players.
Should people REALLY be up-tight about this way of Multi-Classing, we could always limit the number of Encounter Powers etc - to the basic Number available at each level, all the Multi-Classed Character would gain really would be a greater choice. I personally don't like this, as its watering things down again IMO.
HOWEVER - if I were to instigate the 'Old-School' Multi-Classing I would reinstate certain Racial Limitations. Thats not Level Limitations, but rather WHAT Classes each Race may Multi-Class in - Mainly for 'flavour', for example with the Devotion and Piety a Paladin would need - I really don't see a Palading Multi-Classing in this way as Logical.
Just some random musings.