My comments will be in italics, either after someone's answer or after the completed answer (or both):
Q: What needs to be preserved from older editions? Player/DM relationship?
Monte: The core mechanic of #dnd is: player says 'I want to X' and DM responds. Therein lie the stories.
Mike: Offering a wide variety of options so every player can play the way they want to.
Jeremy: The game being a toolbox for players/DMs to create stories together. And fireballs.
There are certain themes that stick out, because they come up as answers to other questions - options for players and DMs and toolbox.
Q: How can we achieve balance in such a modular, flexible game?
Jeremy: What's important to know is that module approach is a spectrum of play styles.
There's a baseline game that provides the foundation. From there, you add on what you want. The seeds are there. (so there is a core game, to which you can add complexity. Basically it’s houserules and they are giving you the houserules to choose from)
Monte: For example, the basic game fighter might have specific level-bases abilities. Things that every fighter has. If you decide to get more customized, you can swap standard abilities for more complex, optional abilities. These are the kinds of things that feats do now. But the complex stuff is balanced with what's in the core. One character is more complex, but not necessarily more powerful. (you can go with the default fighter progression, or you can tweak it to more of the concept that you envision - which I shall assume is similar for all the classes)
Jeremy: The DM should be able to create the experience that their group wants. The players should be able to choose their level of complexity, and have it work no matter the options chosen. (this is tough, because from my experience, the DM sets the tone and style of gameplay, not the players)
Mike: You can see expressions of character types that are found in other editions. (This I bolded, because it plainly states you are NOT going to be playing older edition characters, but MAY be able to recreate the feel of one)
Monte: The DM says: we're using grid, mat and minis. The players can then choose options that match the DM's style. (see, now this is in conflict with Jeremy's answer just above this one - Monte is saying what I said, the DM sets the tone and style of gameplay, the players then chose options to match - there might be some disagreement in the team as to who leads, the players or the DM - but us OSR guys already know that answer)
Mike: If we get this right, everyone is sort of playing their own edition of the game. All at the same table.
This is going to be one heck of a win if they can allow a group to field a party that is “sort of playing their own edition of the game. All at the same table.” Rumor has it they are aiming for a GenCon 2013 release for this. I might have to actually attend for the first time in 20 years ;)