My feelings on the new 4e cosmology are decidedly negative. At the moment the changes seem decidedly destructive and what many things have been replaced with now seem shallow and poorly detailed. Much of it seems like change for the sake of change, and the reuse of many terms in different wrapping for 4e only makes it seem all that much more wierd to me.
Archons have gone from gnostic inspired celestials of strict but benevolent order to elementals with helmets. The yugoloths/daemons have gone from being scheming, manipulative fiends that predate the existence of gods and mortals alike, to 4e "soldier demons".
I see a lot of the trends of certain 3e planar products (Planar Handbook and Book of Exalted Deeds) put on steroids and given front and center billing for 4e. Those books were often panned for being dry, somewhat shallow in their treatment of their given topics, and not always up to snuff on being as well researched on prior lore as they perhaps could have been. 4e so far hasn't inspired me. It feels like pastiche that's going to constantly be compared to what came before it, and oftentimes gave things a much deeper, richer exploration that the 4e sources.
On the 'not researched as much as possible' point, 4e doesn't have to worry about that as much since they've scrapped the idea of continuity with those prior sources. Makes it a lot easier when you don't have to worry about knowing those three decades of sources in order to best work on more stuff. I have to lament that loss of history and iterative expansion on that D&D mythology that developed over the years.
W&M really rubbed me the wrong way, because when it talked about prior editions' material in its justification for changing certain things for 4e, it played rather fast and loose with the facts of some of the sources it talked about being inspired by. I'm sorry, but just because mezzodaemons and nycadaemons were summoned by and served drow in their first 1e module appearance, that doesn't mean it's absolutely logical to make them CE demons in 4e. That's especially true when that original module explicitly has them as not CE and not associated with the Abyss or the Hells. It comes off as a case of scrambling for earlier sources to try to justify a decision that came down to some personal preference. I would have accepted (if not used) "this is how it was before, but we have some new ideas we would like to try with these fiends. So trust us to reinvent them in evocative ways". Instead it comes off as really poor reasoning not supported by the sources it claims as justification.
I hate to point it out, but the 3e planar sourcebooks that had very limited involvement by WotC proper (FCI and FCII) were generally regarded as some of the best books from 3e, and in FC:I's case, one of the best planar books of all time for D&D. If the same trends from the BoED and PlHB continue into the 4e planar material, and a lot of the team from those are front and center in 4e, it's going to be very difficult for me to have anything but a skeptical eye to WotC's 4e efforts (already compounded by lingering frustration with the loss of Dragon/Dungeon, the poor state of Gleemax, potential vaporware on the DDI, etc).
Beyond that, 4e's marketing (and some designers' comments) went almost out of their way to mock and belittle a lot of things I rather enjoyed about the cosmology and planes prior to 4e.
