Okay. I managed to slog through the entire thread. I have some observations.

I don't know that everyone agrees with this approach, but the obvious intent here is to deliberately
prevent someone from creating a character that is
equally good at being both a fighter and a wizard. That's not an accident - it's on purpose.

The only way that the above would be acceptable is if the character in question is both
not as good at being a fighter, and
not as good at being a wizard as the single-classed characters. This is Third-Edition multiclassing. And, let's be honest, being crappy at two jobs is something nobody wants.

What some people seem to want is to be as good a fighter as a single-classed fighter, and as good at being a wizard as a single-classed wizard. This is blatantly, and categorically, utter munchkin crap. You shouldn't be able to replace two characters.

One of the key benefits seems to be that the multiclass feat "unlocks" the class-specific feats.

General cross-training is probably the province of general feats.

Some of these feats give you more of the class flavor than you realize.
In Fourth Edition, what defines a rogue? It appears to be "training in the stealth and thievery skills, sneak attack, and the ability to choose rogue-specific powers and feats." By contrast, a 3e rogue was defined by sneak attack, a massive skill list, and tons of skill points.
I started thinking about the way to make various concepts. And it seems to me that general feats may help with it as much as multiclassing does. For example, we know (or can assume) that there are feats that grant proficiency in weapons or armor that your base class does not. There are also feats that grant skill training in skills your base class does not. There may be feats that allow the character to gain the ability to cast rituals, use cantrips, take combat styles, or various other class abilities. And those feats may be needed to make multiclassing concepts work as much as the actual multiclassing feats are.
Want to dabble in magic, or play a former apprentice wizard without taking the arcane initiate feat? Maybe there's a feat called "cantrips." Heck, maybe you'll play a rogue who takes skill training (arcana)
and cantrips. And then he takes Arcane initiate at 2nd-level. Sure, he hasn't boosted his rogue skills with feats, but he's got some pretty nice magical "oomph" when he needs it (kinda like the Grey Mouser).
As a side note, given that there are feats with prerequistes of 4th, 8th, and 10th level, I suspect we'll be getting feats at 1st and every even level thereafter. That jives with the 10th-level character with 6 feats (1 at 1st, 2nd, 4th, 6th, 8th, and 10th), assuming the character in question wasn't human (since it seems humans get an extra feat).