I'm going to jump in here, without reading the whole thread. Skimmed, but not every line or every post. Yes, I will be that guy.
The simplicity of 3E multiclassing is definitely appealing. It takes up no rules "brainspace," it's a straightforward concept, and easy to implement.
Having a whole "multiclassing table" for every class is onerous, confusing, and would defeat the purpose of simplifying the rules. No alternate tables.
My thought: why not combine 4E and 3E multiclassing a little bit?
In order to level up in a second class, you must meet a Prerequsite. That Prerequisite would be the Multiclass Feat. That Feat gives you one Ability from the class.
On the one hand, this means once you gain the new class the feat becomes "wasted." However, this allows the slow(er) progression of class abilities without needing to create whole new tables!
I'm a Fighter2, and decide I want to "dip" into wizard. When I level up to 3rd-level, I take the Multiclass (Wizard) feat.
Luckily, the Wizard class has already been organized into discrete Abilities at Level 1: Spellbook, Arcane Magic, Cantrips, and Arcane Knowledge. I can only choose one, and since Spellbook and Arcane Magic are interdependent, it's useless to choose either of those. I choose Cantrips.
So, I'm a Fighter3 with knowledge of three minor spells from the wizard's spell list.
When I level up to 4th-level, I can choose to take a level of Fighter or a level of Wizard. I've spent a level studying the ways of magic (verisimilitude), and I've had a level to enjoy partial wizard abilities (mechanical balance).
I become a Fighter3/Wizard1 with a "wasted" feat that allowed me to multiclass, but the full abilities of a 1st-level wizard and 3rd-level fighter.
This is also symmetrical.
I'm a [/b]Wizard1[/b] who takes the Multiclass (Fighter) feat at 1st-level and choose between Combat Superiority and Fighting Style. Fighting Style depends on Combat Superiority, so I choose Combat Superiority and know the Deadly Strike and Parry combat maneuvers. (However, I do not have the Armor or Weapon Proficiencies... yet)
At 2nd-level I take the level in Fighter (gaining Fighting Style and all Proficiencies), and continue to take Fighter levels.
At 4th-level I'm a Wizard1/Fighter3 with the same abilities, the same number of useful feats, and the same bonuses as a Fighter3/Wizard1!
Perhaps Weapon and Armor Proficiencies should be called out as "Abilities" that can be chosen from?
Of course, this idea would require some revising of the math (possibly attack bonuses, Ability Adjustments, maybe Hit Die, etc.) and some organization (making sure that Class Abilities are super-clear and work semi-independently), but I don't see that as a bad thing: it forces the design to be clear!
In my head, some version of this would be the ideal multiclassing.