Before I start commenting specifically, here is my assumption -- and it is just an assumption, albeit based on stuff I've read on Paizo's boards -- about the minimum JB wants for B-C in Pathfinder.
I think he would want a DM to be able to take a 3.5 stat block and run it, easily, on the fly. Absolutely no conversion time required, pre-encounter.
So the question becomes, is a change so radical that a stat-block just wouldn't work? Does this system fail that test?
Arguably, and I suppose obviously, it depends on just how free-wheeling the DM is willing to be when it comes to interpreting the stat-block. If a DM (and JB) is willing to accept that, "Hey, the fighter 8/wizard 8 in this 3.5 stat-block is significantly weaker than the fighter 7/wizard 7 in the PCs' group," and just play the stat-block as written, it's got a shot. JB has shown that he'll go out on that limb to at least some length, because Pathfinder PCs (core, anyway) are noticeably stronger than 3.5 PCs.
Me, I'd accept that tradeoff for better multiclassing rules. In a heartbeat. (Either tradeoff, actually. Either weaker stat-blocks, or some conversion time.)
1. We need a unified spells per day chart. I think most people could accept it.
We need it for this system to work? I agree, and I agree that most people would accept it in principle.
2. I also think stacking Base Magic Bonus is a good idea.
I absolutely agree here, but I prefer Base Caster Level. This is the number you're going to use when you make a "caster level check," right? Plus, "BCL" is easier to say, and not as easily confused in speech with "BAB."
3. We clarify that BMB is just how familiar you are with magic, not how good you are at using magic.
But it is how good you are with magic, isn't it? Clarify that it encompasses both, as familiarity certainly informs skill.
4. This needs to be fairly flexible so that people who want to convert shugenja, wu-jen, duskblades, and spellthieves can do it. Classes with their own mini spellcasting thing, like assassins, should probably just stay stand-alone.
Does "mini spellcasting" include rangers and paladins?
Everyone gets spell slots from their caster level.
"Caster level" or "BMB"? I assume you're using them synonymously, which is another argument for "BCL."
You can fill those spell slots with any spell you have access to from any class, but instead of requiring an ability score of 10 + spell level in the requisite ability score to cast a spell from a given class, you just need to have a number of levels equal to the spell's level.
This is obviously the key to the system, and I like it. However, I don't like divorcing max spell level from the stat. I think this needs to be the lower of either.
If you're worried about MAD, screw it. Multiclassing is a choice. If you wanna be competent at two distinct skill sets, have the raw talent to back it up. Also, remember that, e.g., 5th level slots that couldn't be filled by a wizard 8/cleric 8 with a WIS of 14 can still be filled with wizard spells.
(As an aside, I assume bonus spells would only apply to the class with the applicable stat?)
So if you're Fighter 14/Cleric 6, your caster level is 13, but you can only cast 6th level spells. If you're Wizard 14/Cleric 4/Druid 2, you can cast 9th level wizard but only 4th level cleric and 2nd level druid spells. If you're Fighter 18/Wizard 2, you get a lot of spell slots, but only 2nd level spells.
Sorcerers get bonus spell slots, and can't prepare spells, but they learn 2 spells per level they can cast spontaneously. Beyond 3rd level, at least one of these spells has to be of a level below your highest spell level.
Okay. Starting to get a little creaky, but I get it.
Much. IMO, if anything more radical than "multiclass feats" can make it into Pathfinder, it'll have to be something like this.
How does it interact with, e.g., the mystic theurge, which is, after all, a core prestige class? (If the answer is, "The mystic theurge and similar hybrid PrCs become redundant and get struck from the rules," BTW, I'm okay with that. Not sure JB would feel the same, natch.)