In general, I'm pretty pleased with 4E's revisions to the cosmology. As a longtime Planescape fan, I don't make that statement lightly, as I still have a big ol' chunk of nostalgia for the Great Wheel. But way too much of the Great Wheel cosmology was either not worth visiting for most adventurers (the Good-aligned planes, generally speaking), or extremely difficult to visit (the elemental planes, equally generally). The new Elemental Chaos and Astral Sea give us much better adventurer access to the same basic ideas that the old cosmology gave us, and have the additional advantage of unlimited space for expansion (Astral domains are just as good as demiplanes for justifying whatever bizarre location you might want or need).
All that said, I have to say that I don't like the new Hell. The Abyss has the appropriate feel to me in the new cosmology: it's the festering boil at the bottom of the foundation of all creation, and demons are the horrors that spawn out of it and want to spread its corruption everywhere. Awesome; where do I sign up to fight that?
The Nine Hells of Baator, on the other hand, are an astral domain, which puts Hell...up in the sky. I'm not religious, but for anyone who even grew up in Western Christian civilization, this seems weird. Even weirder, Hell is a big, round ball of rock in the sky, which makes it unmistakeably a planet. The Manual of the Planes even describes it as such, and that's where I officially throw the red card. Admittedly, it's a basically hollow planet, with all the appropriate "caverns and rivers of fire" that Dante taught us to expect from the place, but to anyone with even casual contact with science fiction (and I'm going to work from a stereotype here and assume that that label applies to literally every single person reading this post), other planets filled with monsters are officially the homes of alien species, not theological metaphors for the corruption inherent in the human condition.
My point is that 4E's Hell is placed just fine from an "adventuring logistics" perspective, I can get there, I can imagine why I'd want to go there, and there's a wide range of monsters that I can fight there. But in the back of my mind, I'm not in Hell, I'm on Planet Baator, and the epic nature of the adventure falls apart for me. At least part of this impression is based on Baator being just another astral domain, instead of something uniquely horrible, like the Abyss.
I therefore propose a new (but not terribly revolutionary) location for Hell in 4E: the center of the world. The Abyss should clearly continue rocking the "creeping dissolution of the foundations of creation" thing that it's doing, and we'll give the devils of Baator an "imprisoned schemers trying to get out" feel. Take a stroll with me to the new digs:
As the Dawn War began to wind down, Asmodeus became convinced that the gods would in fact defeat the primordials, and turned his attention away from advancing the war effort and toward profiting from it as much as possible. With both sides weakened by the fighting, he figured an assault on the world might get him the upper hand in the form of a power base from which he could wipe out the last stragglers of both factions. What he didn't count on was the world's own defenses, in the form of the primal spirits. Everything went according to plan until his own forces, including him, were swallowed up by the World Serpent and became trapped deep, deep underground.
Even imprisoned, Asmodeus is still a deity, and has shaped his environment to his liking, creating the nine layers we want in a Hell. His malign influence has also crept up into the Underdark, turning the duergar into a race of devil-worshippers. Perhaps most importantly, the devils being trapped gives us a justification for the deal-making and power-brokering that we expect in our devils. Infernal-pact warlocks exist to allow the devils to exercise their power outside of their prison. Devils can be summoned by mortals through ritual magic because they need outside magical assistance in order to manifest outside of Hell. Devils want to corrupt mortals because enough fellow prisoners might make the prison crack a bit from the strain.
But most importantly, it makes the devils a race that once lived in the sky and now rot under our feet, and that symbolism matches our expectations better than a planet ever could.