Did 4E ever actually have a time when it had hardcovers coming out that fast? Not denying it, but I wasn't aware of it. The sad thing is, I think 4E's pacing would have been pretty solid, release-wise, if they hadn't been putting out the X Power books (all of which would have been better off as just being more articles in Dragon, imho).
June 2008: PHB/DMG/MM
July: None, but supplemental like char sheets and DM screen
August: Forgotten Realms Campaign Setting
September: Adventurer's Vault
October: Forgotten Realms Player's Guide
November: Draconomicon 1, Martial Power
December: Manual of the Planes
January 2009: Open Grave
February: Dungeon Delve
March: Player's Handbook 2
April: Arcane Power
May: Monster Manual 2
June: Eberron Player's Guide
July: Eberron Campaign Guide, Divine Power
August: Adventurer's Guide 2, Dragon Mag Annual
September: Dungeon Masters' Guide 2
October: Primal Power, Draconomicon 2
November: NONE
December: The Plane Below.
I'd say that out of 18 months, they released 22 hardcovers. That's a helluva design and development schedule! (and that ignores DDi and Modules)
To be clear, I personally reject arguments to the tune of "If they put out less stuff, it'll automatically be better balanced!". Every TT RPG I've ever seen which attempted balance or claimed it is pretty much prima facie evidence to the contrary - there should be a link, logically, rationally, sure - but in practice, whether stuff is balanced or not comes down entirely (imho/imex) to three things:
I'm saying if they don't rush to put on 22 hardbacks in 18 months, they might have a chance to better playtest it.
Remember, a bad module can be fixed or forgotten. A bad rulebook lingers in the ruleset forever. If they release fewer rulebooks and spend more time ironing out kinks, we'll have a better ruleset as it begins to expand and mature.