For all the talk of making things different, and making a distinction between demons and devils (and more comment on that issue later), I see very little actual change beyond some surface gloss. Demons in the article, outside of having some nametags shifted around and Tharizdun's name being invoked in their origin to give some sort of old skool street cred... they're virtually unchaged in anything impacting their use in a game. The same thing for devils, but more so.
Without knowing anything beyond what we've seen to know the full scope of what's going on, it's somewhat depressing to suspect that the late 3.x planar work, with FC:I as the standard for such (and probably the single best written 3.x book, period) is apparently getting tossed, with a large chunk of its material being unfit for translation given the planned changes for 4e (Malcanthet, Grazzt and other Abyssal Lords, posterchildren of the Abyss, now don't fit the 4e scheme of what strict role demons are supposed to fill).
I also see no mention of the other major fiend races (though I strongly suspect we'll see more as time goes by). Demons and devils being two monolithic sides of the lower planes was a 3e creation that really lacked a grounding in much of its earlier source material. The yugoloths were every bit as powerful as their LE and CE metaphysical neighbors, and arguably had more influence in many ways (and their wayward brother "race" the demodands were really the true powers in much of Carceri). However they had virtually no elaboration in 3e because they weren't in the 3e MM (though originally planned for inclusion they were cut for space reasons and not wanting to have too many planar monsters in there, at least according to Monte Cook). Compared to the absolute wealth of detail on them in 2e, 3e's design structure early on meant that we saw relatively nothing on them till fairly late in the game, and with no FC:III ever written despite frequent requests, they seem like they'll be passed by during the edition and this lack of relevance by omission looks like it might carry over into 4e.
That's really a shame if that's the case, because it risks removing a lot of depth and complexity from the planes, which is often a major reason -why- people like planar elements in their campaigns. People like different shades of evil, sometimes overt and sometimes subtle differences in the hows and whys behind the fiends. We certainly have that between demons, devils, 'loths, demodands, night hags, hordelings, and myriad others, but the hints on what 4e will have makes it seem like we'll have about the same things we had before in most aspects, but with many races removed from the mix, and without the depth and grounding of previous lore. I'm hoping that this isn't the case. Please, I want to be proven wrong here.
I've made my own small contribution to the body of planar lore, plus the gobs of material I've posted online because I enjoyed it and enjoyed writing it, and I have to feel like the rug is being yanked out all of a sudden for problems in the canonical baseline that I've never seen. It's depressing to say the least.
We all like developing and expanding upon the body of material that came before, and its been developed by some amazing writers over the years since the first Manual of the Planes gave the first real solid take on the planes. But we can't easily riff off of that body of lore because it's being changed for reasons I can't solidly fathom based on what we've been told thus far. It also makes trying to pitch planar based ideas for Dragon/Dungeon rather difficult because you have no idea if your ideas have any place or relevance in the great unknown that is to come in 2008. Hell I'd love to have even some minor part in developing the 'loths for 4e, if they'll exist in 4e, given what I've done before (but I don't know what my chances are of that).
But stepping back and looking at the rationale behind the changes, and why I think it's a bit much and why I think it's missing its target audience, here's one thing. For anyone not using the planes, this doesn't really affect them at all, but for those of us who really do enjoy the planes in D&D, a lot of these changes are -among the people I know in the community- being looked at with a mixture of quickly fading optimism and cold silence. When 3e FR random retconned its cosmology, the reaction was pretty severe and largely negative, and that was only one setting. This risks invoking the same reaction but on a larger scale, and from the very people who you're trying to win over: the people who enjoy the planar elements of D&D. Please realize the extent to which you honestly risk alienating these folks.
I can still be won over, but it's a -very- hard sell when you're dismissing so much of what has been built up over the years, for stated reasons that I don't see, or that come off to me as marketing talk, or after-the-fact justifications rather than longstanding issues that called out for solutions for years that are only now being addressed.