I'm a little nervous about part of their answer to #3.
The "class abilities that advance only in a particular class" is what lead to "full caster progression" PrC's, and "back-fill" feats, and other ways of never emerging from your initial chosen class.
It's very contradictory to say "Multiclassing is wide open! But you will loose your best abilities if you even slide a little bit into some other class. So don't."
I'm a little nervous about part of their answer to #3.
The "class abilities that advance only in a particular class" is what lead to "full caster progression" PrC's, and "back-fill" feats, and other ways of never emerging from your initial chosen class.
It's very contradictory to say "Multiclassing is wide open! But you will loose your best abilities if you even slide a little bit into some other class. So don't." If Parry is something I want for my character (regardless of if they're a Fighter or not), why should I have to be a Fighter to do it effectively?
I think specialties might be able to help with this, though. No reason a given specialty shouldn't be able to give you Fighter-equivalent Parry even if you're a ranger or a paladin or a sorcerer or whatever.
I think the other way to help with this is to ensure that classes have decent higher-level abilities that are different from lower-level abilities. A fighter that just gets MOAR DICE isn't going to give someone a reason to stay in it. A fighter that might end up giving, like four attacks, with immense damage bonuses, at higher levels, is more likely to be tempting to stay in for a while. That's part of what spells offer: 8th and 9th level spells are awesome enough that you don't want to give them up, except for something equally as awesome. High-level fighter or ranger or whatever abilities should be the same.
I prefer Specialty-based multiclassing (aka the 4e feat-based MC) or flat-out Hybrids (as in 1e/2e and 4e) to the level-by-level multiclassing. Between Arcane Dabbler, Healing Initiate and Martial Training, you can almost see a glimpse of feat-based MC already (but Martial Training needs to give you one MDD, if you don't have one).
I think 4e's "3 kinds of Vampires" approach works well here. Give some specialties. Give some hybrid rules (which just really sounds like alternating levels). Give some level-by-level multiclassing. No reason to limit it to one way, and the more ways there are, the more ways you can pick up exactly the kind of multiclass abilities you want, without piling on the others.
The basic game will not have specialties or feats. Each class will have a specialty built in as a class feature. At the standard level, you can choose a specialty, or create your own picking feats a la carte.1. I'm glad basic characters and advanced ones will be compatible and on the same overall power level. They do make it sound like feats won't be optional, though. They make it sound like basic characters will just use specialties, while other characters can pick their feats a la carte. We've known that about specialties for a long time now, but I was always under the impression people could remove them entirely. I don't mind it, I prefer having feats in the game, but those that don't might find that upsetting.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.