I very much do not want to see a default setting built into the crunch. A bit of fluff to help new people - fine. The 3e default pantheon was a waste of page space for me, but I could see why they did it, and it was easy to ignore. But do not built anything of the sort into feat names or anything else.
(I'm a little torn on spell names. Mordenkainen, Bigby, Otiluke, Tenser - these are part of D&D's history. Arguably they now transcend the Greyhawk setting they originated in. But a little goes a very long way - let's not introduce any more. Though I do have a soft spot in my heart for Abi-Dalzim, it must be said.
I do see the point of a sample setting for beginners. These can really help fire the imagination - by giving examples of what's possible, and even by their flaws! Mystara, for example, has some great stuff in it, but also some dumb stuff that encouraged me at the time to do better. But fluff only, please, and emphasize that it's only an example. It doesn't need to be a $40 book - a booklet that comes in the boxed set will be fine, or if absolutely necessary, a couple dozen pages at the back of the DMG.
I do not ever want to see the Dinner Plate again. (My group's derisive name for the so-called 'Great Wheel'.) I have despised that thing ever since I first saw it in the AD&D DMG at the age of 11, and I've met plenty of people from the same era with the same reaction. I realize this is likely a losing battle, and that they will feel the need to mention it somewhere. But PLEASE, WotC, don't build into the core rules!
I actually rather like the 4e cosmology - one of the few things I thoroughly like about 4e, in fact. I think it's a distinct improvement over anything that came before. But I don't think it should be pushed hard either. The core rules should include only the most minimal of setting assumptions about the planes. It's impossible to have none, of course - if there's an Astral Projection spell in the book, they have to say something about an Astral plane - but I think there should be light touches only, with some options presented in the worldbuilding section of the DMG. Leave the rest for a Manual of the Planes.