First off, I am not reading through 550+ posts before commenting here. I'll forget half of what I want to say, and I don't have the time to go through all those posts anway.
From what I can tell, it looks like it has potential. Having played enough pre-3e and 3e, I've seen how D&D's multiclassing worked in the past. Even if these rules turn out to be bad, the designers are addressing one of the weak points in the game.
Old-school multiclassing was restricted to demihumans only, humans were stuck with that horrible dual classing stuff in 2e and maybe earlier. Even then, the rules had limited numbers of multiclass options (given racial class restrictions), and there was only a single triple class option available, the elf (or was it half-elf?) fighter/thief/mage. While XP slowed down a bit, the characters tended to be pretty strong, and always got the best THAC0 (usually fighter) and saves.
Then 3e had its problems with dipping. I'm sure there are those who absolutely loved 3e's multiclassing (and I do agree it was an improvement over the old stuff) who'll disagree, but there were problems. Like taking a level of fighter just for full armor and martial weapon proficiency. Like taking a level of monk with a divine caster for the Wis bonus to AC. Like taking a level of paladin for the Cha bonus to saves. And so on. Yes, there were the XP penalties for doing that, but builds that worked with a favored class got around that. In addition, multi-classing was a bad option most of the time for a caster, simply beause of lost spell levels. Multiclassing in 3e was good when it involved 2 or 3 fairly compatible classes that worked well together, but the system was a bit too abusable, and penalized spell casters disproportionally.
This doesn't look too bad. In essense, it seems to go back to the tradition of two classes only while avoiding the overpowered mutliclasses of yore. There's no need for a jack-of-all-trades multiclass build in the game, since D&D has long about teamwork and having a party of specialists who wrok together to overcome challenges.