This is actually one of the other problems with epic-level play. The vast majority of epic foes are extra-planar is nature. Although the outer planes can always be a threat to the PCs' homeland, epic-level play tends to involve the PCs leaving the locations and NPCs to which they have developed attachments. That's a major barrier for the games I tend to play/run.
I also see the ghettoization of epic content as kind of a problem. KidSnide, your observation's spot on. I run a pretty social game, so many of my players are interested in relationships with NPCs they've been hanging around since the early levels. Not just romantic relationships, mind: mentorships, rivalries, friendships, even things like playing matchmaker between NPCs. Stuff that goes on in between adventures. As designed, epic tier is meant either to take place somewhere outside that world or to draw the players' attention to the world-shaking threats it represents, discouraging them from spending casual time in a peaceful setting.
I can see where these comments are coming from, but I don't entirely agree. I fully agree that a game which simply switches location, from the world where play has taken place up until now, to the outer planes, won't work for engaging the players. (In my view, the city of Union in the 3E Epic handbook is a classic example of this sort of failure.) But Epic needn't be like that.
I know from my own experience running high-level Rolemaster that it is possible to run an epic-style game - with wahoo magic, with ancient dragons, angels, gods and demons as the principal adversaries, with other planes as the site of many of these encounters, etc - in a way that is tightly integrated with a mundane homeland that the players (and therefore their PCs) care about. But what this requires is seeding that homeland with these connections to deeper mythical forces, and the other planes where they live and play out, from early in the game.
I think there are a lot of permutations of theme, tone, and relationship of epic play to the rest of the game and setting. Some combinations are likely to work better than others of course.
All of them IMHO generally involve epic play being tied to a fairly deep story is what I'm getting out of the whole discussion.
Not only a deep story, but one which is linked to the mythic/other-planar features that (given what WotC is actually publishing) are going to be the core game elements of epic tier play.
This means that you are right when you say that some lower-level adventures will work better than others for seeding Epic play. Not every god, every artefact and every campaign backstory is equally well-suited to the task. I think the lack of a discussion of these matters is one of the weaknesses of the existing GM advice for epic play.
It's also interesting that (as I read it) the Plane Above appears to cater to multiple approaches to Epic tier - the discussion of the gods, the compact of heaven, venturing into deep myth and so on all seems aimed at the sort of epic play we're discussing in this thread, whereas the Outer Isles and the Githyanki pirates seem to me to be more relevant for an Epic tier which is just more dungeon crawling or adventure without a strong story integration into what has gone on at lower levels. Again, that book could probably be improved by talking a bit more frankly about the different sorts of play its contents are suited to supporting.
