Personally i really dont think the multiclass system is that different either. If you take out the favored classes with regards to XP penalties then its pretty much the same.
I know I'm going to regret this, but, I just gotta say, "buh?"
Umm, let's take it by the numbers shall we? In AD&D (either 1e or 2e) multiclassing was done at 1st level, could only be done by non-humans, had very specific limitations on what combinations could be played and to what level. Multiclass characters, by and large, were 1 or at most 2 levels behind the rest of the party until double digit levels where the xp tables stopped progressing and flatlined. Abilities were as the level of the given class, with some restrictions. Couldn't wear armor and cast MU spells for instance. Hit points were averaged between the classes. Everything else was pick the best of either class - THAC0, Saves, exceptional strength for fighters, etc.
Multiclassing in 3e can be done by any character with almost any class (some alignment restictions apply - although, you can still be a barbarian paladin, you just have to give up some stuff

), and the total character level will never exceed the rest of the party. Hit points are cumulative, there are no level limits, BAB stacks and there are still some restrictions for wearing armor.
So, instead of having a 7th/7th Fighter/MU in an 8th level 2e party, you have a 3/4 (or some version thereof) Fighter/Wizard.
How is that even remotely similar? Other than the fact that both characters have two classes?