generally like
I generally like the new cosmology.
Running a heavily planar campaign, currently on hiatus pending a convert to 4e, there are a few things from the new cosmology I will not take, mainly to remain consistent. Primarily, the campaign deals a lot with the separate Elemental Planes, in an orrery cosmology where the planes rotate around the Material with one being dominant at any given time. This is very important both for the campaign backstory/main arc and the player characters' experiences. But ok, no problem, I can keep them separate. In fact, having them collapse into an Elemental Chaos could be a perfect climax for the campaign, depending.
I have some more issues with changes to individual planar creatures. I'll need to keep some single-element elementals around. Yugoloths remain separate. A big pain will be genies, which are one of the main groups of antagonists for the game. The MM only has efreeti; also they are epic level challenges, which doesn't really fit with the previous events of the campaign where the party has defeated efreeti at level 9. Perhaps the MM efreeti will be the nobles and common genies will be high heroic into paragon level threats.
But that's pretty much all specific to my campaign conversion. Outside of that, the one thing I feel a little ambivalent about is the removal of the Ethereal. Basically it got split into the Feywild and Shadowfell. I have nothing against these per se, and in fact I like their flavor - they are less boring than the pure Ethereal, for sure. I guess I just liked the mechanics of the Ethereal which seem to be gone. Like, when an eladrin fey steps through the Feywild, he just teleports and the fact he's going through the Feywild is just flavor. There isn't any real allowance for a phase spider hanging out there to intercept him, or an ethereal solid barrier. (Not a huge deal, these things can of course be created with the DM wand if there is a need for them.)
Maybe it's just that there is no longer a "neutral" ethereal. For instance, the jann in 3e can use ethereal jaunt, which I see as showing their unique (for genies) tie to the Material plane. But would a 4e jann travel through the Feyworld or the Shadowfell? They don't seem particularly fey nor shadow but I guess I'll have to pick one.