Something that I think isn't being considered fully by detractors of 4e multiclassing is the obvious slower power progression between levels, versus 3e. Those "wasted" feats will actually grant you some pretty powerful abilities that are usable over the lifespan of the character (sleep has been the obvious example, but given the choice of powers between 1 and 10 with the "swap-out" ability, I'm sure there are even better examples in the book).
The ability to keep updating the powers as time goes on is what makes the difference to me - retraining may allow this for single classed characters, but what if it doesn't? We know wizards can swap their dailies, but what if that's all they can swap on a regular basis, with retraining being more limited? In that case, the ability to use any of the encounter powers at one point or another would be a pretty significant boon in and of itself.
All in all, I'm actually liking what I'm seeing, because the 1e and 2e multiclassing rules struck me as not aiding their original purposes. 1e multiclassing was too powerful for the low levels, and totally stunted for the high. It was a game system where if you knew the DM was running for more than 6th or 7th level, no one in their right mind ran a demi-human, and if you knew the DM was probably only going to stop the game at 6th or so, then no one in their right mind ran a HUMAN. Mostly, in our games, it was the latter, and the game petered out at 5th to 7th level, when another player started getting the drive to DM something. Instead, if you were in for the long haul, you played a human with at least an 18 in the score for the class you were going to dual-class into later...
In 3e, among non-spellcasters, it practically encouraged multiclassing. The barbarian for the occasional rage, 1 or 2 rogue levels for the rapid skill increase in your main skill set (or factotum for the inspirations also!) and evasion, 2 levels of fighter for the feats, some swordsage or warblade for a splash of martial maneuvers and weapon switchout mastery, and maybe some levels of duskblade for the will saves and a few useful melee powers. In all, the only thing that would hurt hit points is the rogue, you'd have top notch BAB, and a LOT more skill points than the fighter, paladin, or barbarian.