I agree that HeroWars gives a good handle on how to approach Epic play. I think there is also some good stuff in The Plane Above - I posted on this when that book came out.Playing that level of game is one thing, but creating the environment where players are ready to take on that responsibility and have something they believe in so strongly that they want and need to be 'god' of it is more difficult.
I don't like the way PCs hit level 21 and suddenly its 'WHAM! You are now godlike!' Especially if at earlier levels you could essentially bumble through by killing everything and picking up the treasure.
To make it work you need to be sowing the seeds of plot and drama and character way before then. Like a PCs first love has been turned undead (when he was, say, level 8). He's tried all kinds of quests and rituals and whatnot to get her back, and failed and failed and failed. And finally (at level 21) he goes 'Screw you Orcus, things just got personal.'
And the party quests and battles and plots and eventually kills Orcus, and undeath no longer exists for a while (until some author with a vivid imagination goes and writes Frankenstein and he seeps slowly back into the world). But what really happened is the hero got the girl - killing Orcus wasn't IT. It's just how the story to get the girl played out.
This is the way I see Epic level. PCs can get stuff done by rearranging the concepts that create the world. But the responsibility lies with the GM to provide meaning to that world such that they have personal stuff they need to get done.
These ideas don't come from running or playing D&D, btw. These are Herowars ideas, but I think they fit the discussion.
The only part of your post I'd quibble with is where you say "the responsibility lies with the GM to provide meaning". I think the GM bears the responsibility to make this sort of meaning possible - by doing things like killing off the girlfriend at level 8 - but the players also have to be prepared to invest in the game and in their PCs. Luckily 4e has (in my view) a lot of easy ins for the players - the Raven Queen, the split between the branches of elvenkind, a good variety of demons and devils, an interesting god of civilization combined with a post-civilization/points of light gameworld, etc. These are also features that integrate the planar bias of Epic (which I agree is very obvious) with a concern for the PC's own world (which I agree is an important part of emotionally engaging the players).
In a game that focuses on gods and themes that (at least to my eye) don't provide the same easy ins - eg Pelor as a prominent god (where's the drama?), exploring dungeons on the Outer Isles of the Astral Plane, hanging out in Sigil, mapping the Elemental Chaos - then I can see that Epic Destinies and the Epic Tier might be harder to motivate and extract engaging play from.