That's the only one of the seminars I've watched, so far. And I agree with you. It was highly enjoyable.
One of the most interesting aspects was that there was a confirmation that, to start, there isn't a core D&D world. Right now, there is only a world bible for the Forgotten Realms that is divorced from the rules.
And that each individual setting will have its own world bible. (I like how Jon noted that Mike [Mearls] is a big fan of Greyhawk.)
Another great, theoretical, comment/question was his idea of releasing a set of core rules that didn't refer to any world or have any art. Would that be something gamers would buy?
I think I'd be interested since it wouldn't come with any prepackaged notions about setting, which would allow me tomore easily build the setting I'd want to run. Yet, I would hope its graphical design is interesting enough that it wouldn't be like flipping through a dry textbook.