D&D's cosmology and how it will fit with D&D Next and previous editions' worlds. You can read this article without a sub, so check it out first.
WotC's experiment with a streamlined 4th Edition compatible universe was nice in many respects, but they tried too much to ditch elements of older settings that they thought were not worth carrying over. Now I am happy to see they're backpedalling (a little) on the cosmological changes they made and are even acknowledging areas ignored up till now.
The elemental planes get a hybrid treatment. Looks good.
"the elemental planes will be divided into three basic rings that surround the prime material plane. The innermost ring consists of the border elemental planes. These regions are like the regular world dominated by a specific element. The border plane of fire is a land of ash deserts, billowing volcanoes, and lakes of lava. The next ring out consists of the deep elemental planes, which are areas of pure, elemental energy much as the elemental planes were portrayed in the Planescape material. Finally, the outermost ring is the elemental chaos, a region of pure, fundamental elemental energy."
Wizards put a lot of energy into their 4e vision of Feywild and Shadowfell to retract it. Personally I never used these areas much in my extra-planar adventures and it seems Ravenloft gets to stay put, the poor place having been retconned so much the last couple editions.
"we're treating the Feywild as a similar border plane between the positive energy plane and the prime material. The dreaded domains of Ravenloft are its opposite number, between the negative energy plane and the prime. Elements of the Shadowfell can become domains within Ravenloft."
Planescape fans can rejoice as their favorite setting gets a second chance.
"When it comes to the outer planes, we're treating Planescape as our default assumption. It's a much-beloved setting and one that's fairly easy (by design) to integrate into existing campaigns. That means the return of the Great Wheel, the Blood War, and other classic elements of the D&D cosmos."
(If it was so beloved and easy to integrate why did the Wheel cosmology have to go?)
Now the biggest surprise of the article is serious talk about Spelljammer:
"The biggest setting change we're looking at concerns Spelljammer. In the past, it incorporated all of D&D's settings as places you could visit. I'm not sure that's the strongest selling point of the setting. In my mind, Spelljammer was an interesting exercise in placing D&D in space. Adding in Faerûn, Oerth, and other worlds muddied its initial vision. It also says stuff about settings that might be fairly jarring given a world's flavor and feel. Not everyone wants the equivalent of spaceships in their campaign, so I think that when we talk about Spelljammer as it relates to other settings, we're going to focus on it as its own setting and downplay its role as the connecting tether between various D&D Next worlds."
So yes, Mearls and company want the planes in "5E" to work well with any setting you play. This is a smart move in my opinion and though the workings of the planes matter little in a casual Greyhawk campaign, this article and many others, plus every action Wizards has taken in the last year from reprints to re-releasing PDFs, shows that they are respecting prior material more this time around.