Just read an older post from Tobold, in which he proposes a form of horizontal expansion model to keep the whole content of the game more alive: Instead of adding more content to the level cap -thus making the old level cap content obsolete- he suggests that the players at the cap could get more beginning class options opened. Like in a racing game, you open up more car options after winning certain amount of races. From basic four (mage, warrior, priest, rogue) you advance to hybrids (paladin, warlock, druid, shaman) from which you advance to heroic classes (dk and so on), which you would start from the level 1 and keep the old content alive for the newcomers to come and enjoy the populated MMO environment.
(Side note: It's funny, but when I wrote my Learning to Play text, both syncaine at Hardcore Casual and Spinks in Spinksville wrote about the same issues which boggle my mind. Great minds and so on.)
What I had bobbing in my mind was the reason why the former end game has gotten forgotten. Why the reputation grind for factions goes completely unnoticed (in WoW), why the instances get forgotten and passed on the premise of a new expansion and how come the designers forget to tie in the expansions (at least in WoW... In EQ2 I saw some pretty neat quest line tie ins leading from one content to another...).
It seems that the vertical expansion route deliberately forgets and abandons the former -"farmed to death"- content in favour of the new one: the entry level to TBC was lv58 (56 if you had a mage to teleport you to Shattrath), and for the WotLK it's lv68 (which is IMO still too OP for properly geared toon...). What especially bothers me is that Pike's example I linked to earlier: Sunwell and Kael'Thas has been neglected by a huge amount of people, and still it's bound to be a great experience -even as nerfed one.
Let alone Mnt Hyjal and Black Temple...
To turn obsolete 'end game content' into something else... you have to device some twists into the former end game rewards and/or add some levels to the reputation rewards which incite the need to visit and do more for the earlier factions in order to advance in the new ones. Currently you can easily advance through the game and forget the faction reputations, and grind only the level cap ones you want to.
What if this wasn't so?
You couldn't gain the much fabled reward enchantment without an earlier faction rep at certain level: the fastest way to gain this would be a mentoring run through an earlier (heroic) instance. Old World endgame instances should be cut into smaller pieces and designed to work the same way as the Outlands and Northrend ones, being fairly easy one shots in normal and challenging at the heroic. The instances granting extra amount of reputation could be even harder than Heroic, scaling to the level of the party attending to it.
The options are there: it's all about the willingness of the design team to tackle this 800lb gorilla monster and make some tweaks in the basics. The game has been dumbed down enough now.
We casual players have brains, too, so the challenges could be something else than just adding twitch into the boss fights.