Races & Classes and Worlds & Monsters aren't just fluff previews of 4th edition. They're previews of presentation quality in 4th edition.
Looking back at the original 3rd edition adventure path (Sunless Citadel, etc) the presentation wasn't nearly as good. Scourge of the Howling Horde is similar. Black and white pages (including maps), a small quantity of mediocre quality art and poorly drawn maps stapled together.
The H (Hero) series will be 96 pages each, the same size as the two preview books. From the press releases, WotC has said they want to go with a 'magazine presentation' style with glossy, full color pages.
Dungeon magazine started this trend long ago, but like all D&D modules these were presented with a primarily gamist design. They're modular and focus on presenting a balanced challenge to the players. Unlike a sourcebook, they provide almost no supplemental support for going 'off the rails', be it to simulate the surrounding world or to flowchart different narrative possibilities. Challenging maps and encounters were the centerpiece, with a little flavor thrown in to accentuate the two (and excellent flavor in many cases, but it was the sizzle, not the steak).
DMs with very little prep time (and have a table that understands the DM's situation) will run canned modules off the shelf. How do they pick which one they want to run? Sometimes it's for a shared experience. Like the classics, they want to run something they can go to a message board and talk about years later. They want to 'capture the moment' in game. Dungeon magazine offered so many adventures that it was difficult to capture this 'shared experience'.
Running just one module is too gamist for most tables to stomach (except maybe in the RPGA), so most DMs will build a campaign to tie various modules together. Again, with so many modules out there it's tough to get that 'shared experience', and in many cases tying the small adventures together can be more work than prepping your own campaign material.
Adventure Paths provide this solution, and from what I've read they're much more successful than stand alone adventures. They are a common shared experience that can be run as an entire campaign with a very gamist "outwit, outplay, outlast" mentality from start to finish.
I think many people want a big magazine style gamist module they can start and end their campaigns with. Paizo is doing this with Pathfinder. WotC is following their lead with the Hero (H) series, followed by the Paragon (P) series and finally the Epic (E) series. The total amount of material to go from 1st to 30th would be 864 pages of adventure.
And in some ways, I see it as a tribute to Paizo's work with Dungeon magazine and now with Pathfinder. Imitation is the sincerest of flattery. Everyone should thank the staff of Paizo for raising the bar and helping WotC produce better adventures. If 4th edition is successful it will IMO be in large part because of Paizo.