I sort of like the general idea - have a short chapter like the 4e "hybrid" chapter that goes through each class and details how it works in multiclassing. And I imagine SOME of the confusion will be solved by having spells and powers scale by "character level" instead of "class level."
But this bird's-eye view of the issue doesn't really explain a lot of the obvious issues with such a system. The biggest one is a point others have already made: to be balanced and useful, your fighter10/wizard1 needs high-level spells, but to be "realistic," it seems weird that he can learn Cone of Cold before he even learns Magic Missile. I'm guessing they're going to have to rely on rules options to please everybody here: for example, there could be an optional rules module saying you can only multiclassing by alternating two classes starting at first level (to simulate hybrids or 2e dual-classing).
But even after they decide where they're coming down on that dilemma, there's still a lot of trickiness to be figured out here. Everyone in this thread is focusing on spell levels, since that was such an issue in 3e, but what about fighter expertise dice? Does a wizard10/fighter1 get 1d6, or does he get something like 1d12? 4d6? 4d12? If he only gets one die he'll probably never be able to use high-level fighter maneuvers like Whirlwind that require 3+ dice - would that be akin to a multiclass wizard stuck with useless low-level spells?
How about a rogue10/sorcerer1 - how much Willpower does he get? Probably less than a level 11 sorcerer but more than a level 1 sorcerer, but what's the formula? Does he get all the origin transformations of an 11th level sorcerer, or just the first-level one, or somewhere in between? How about origin-specific spells like Dragon Breath?
How about warlocks - right now the only difference in combat between a level 1 and level 5 warlock is 1d6 of Eldritch Blast damage (and a few more invocations known, almost all of which are defensive, evasive, or non-combat in nature). If a rogue10/warlock1 gets the same Eldritch Blast damage as a warlock11, is it really worth all those extra warlock levels just for the versatility of more invocations known (when you're still stuck using only 2 of them per encounter)? How about pact powers?
And of course every other class, and every new class, would have an equally thorny bundle of questions to be resolved, many of which would seem to require a bunch of different variables. Now, each of these questions can be answered with careful consideration and playtesting, but I'm not sure how they could avoid it being utterly confusing and 100 pages long when written up into a chapter.