Book layout and presentation.
I think you've hit it on the head. I think if you were to mash all the playtests together, throwing out all obvious contradiction and all very unpopular things (>50% unpopular eg), you might approach what the PHB/DMG will contain. How its layed out, presented, etc is key to its success. There are a couple of considerations:
Initially its the Diehard D&D enthusiasts that will drive sales. People on these boards. We create the buzz. Lose us, lose sales, you make a new printing.
To win the acclaim of the diehards, modularity and its presentation, the ability to Grok, and the amount of options that allow a critical number of us to create the D&D experience we want is the first goal.
Second goal is new blood. New players need to enter and digest enough to get them going without a bewildering array of choices that ultimately paralyze them. WotC can do this a number of ways, but I think the DMG/PHB should be ultimately an experienced players book, with only "ease into it" approach, that is early chapters get you playing with basic rules, middle chapters get into standard game, and final chapters get into modularity. They should release a "box set" or some similar "basic only" ruleset as well. It can be released after DMG/PHB, but should follow fairly soon.
Third, Adventure support. Right out of the gate we need to be hit with a A-List Adventure Path similar to Paizo's wonderful products. Thats tough to do when the ruleset hasn't been finalized, but once the game is locked down and they are down to layout, typesetting, art, etc they need to hit the AP hard. Personally, I'd release the first AP as a free PDF download, to be followed by a compiled print run if it was received well. Adventure support, highlighting the new game, and what can be done with it would be a big draw.
For many of us, the D&D experience is defined by these AP's. A good home campaign is simply an unpublished AP. These multi-level goals/challenges (campaigns) are what defines the game in many ways. 1E in many ways is defined by Temple of ELemental Evil, Scourge of the Slavelords, Against the Giants, and Vault of the Drow (single or compiled adventures). Many 3E games are defined by the APs in Dragon (pre-pathfinder Paizo) such as Age of Worms or Cauldron (or whatever the AP was called).