Just writing that gives me a disturbingly evil smile, because one of the things I've enjoyed most since hitting 80 is soloing heroic Sethekk in pursuit of the Raven Lord mount. Hi there, Syth. Remember the days when you used to mop the floor with my furry ass? I do, and this dish of revenge is being served to you at the proper temperature, courtesy of the frigid Northrend climate. How does it feel to be killed just because you're on the way to something bigger?
I don't know that soloing heroics or raids is currently possible for most classes and specs, but I think the array of mounts to be gained from such ventures is a fairly big draw. A prot Warrior friend of mine has been able to continue her quest for the Zul'Gurub tiger mount without outside assistance, I've been after the giant chicken, and a Rogue buddy spend a portion of his week seeking like-minded players interested in farming Attumen's mount. As far as I know, there are four rare mounts (the tiger, the Swift Razzashi Raptor, the Fiery Warhorse's Reins, and the Deathcharger's Reins) that should be within players' capacity to solo or duo at this point, and you can add a fifth (Reins of the Raven Lord) if you're a Druid or can convince one to accompany you. The Ashes of Al'ar are still well outside the realm of soloing possibility -- and may be that way for a long time due to the mechanics of the Kael fight -- but then again, nobody knows just how overpowered Tier 9 is going to be yet.