I haven't run anything remotely near epic tier. Most of my games happen in and begin and end with the adventure, not the campaign. Additionally, my play group tends to vary adventure to adventure with lots of new players.
That said, I think people approach epic the wrong way.
You can't "win" D&D. But many players try to win by getting their character to level 30. Many DMs think they aren't doing their job if the current adventure doesn't have a path to getting PCs to 30.
IMHO, there's nothing wrong with the system or with players if epic is rarely touched. It is supposed to be rarefied and, well, EPIC.
The failure of Epic Tier is only one of imagination of WotC and players. You have to have epic plots, epic challenges, epic-ly mind-bending settings and cast. There's not much example of this in the literature or in popular media, which makes it difficult for the DMs at home trying to write epic adventures on their own. Maybe Transformers: the Movie where a living planet is trying to eat other planets? The end of 2001: a Space Odyssey where the astronaut is being mind-
ed? (pardon the language)
Another problem: encounter locations get so weird and sprawling it becomes very difficult to adequately represent them in such a way that they work with the tactical combat system of 4E with any verisimilitude.
I've said this before, too, and been criticized with it: because the PCs are near gods and because combat starts breaking down or becoming meaningless, epic should feature lots and lots of roleplaying and skill challenges. An epic adventure should be about (and reward) putting pieces of complex plots into play, directly interacting with the gods as equals rather than trying to kill them, manipulating heroic-tier and paragon-tier adventurers, and traveling the cosmos.
A wacky idea I've had that would take a super-human DM to pull off would be having one party at heroic tier being called into action by a party at paragon tier being manipulated by an epic tier party.
I think the ideal use for epic tier will prove to be for experienced 4E players who created level 20-something PCs for an epic-tier-specific one-and-done adventure. This kind of play simply couldn't work in the first couple years because players and DMs were just too new to the system and needed to gain experience at heroic and paragon. Maybe I'm projecting.
Woof. This has turned into a too-long; didn't read brain dump. I'll stop now.