Three times now, we've been pulling a group together to get the Raven Lord or White Hawkstrider and the same thing has gone wrong -- someone in the group isn't able to enter the instance.
For Magister's Terrace, this is not a huge issue. The requirement is the regular mode quest chain, which can be wiped out quickly by any group able to handle heroic mode -- it just means you run normal mode once (find the spy, click the orb, get Kael's head).
The Raven Lord in Sethekk Halls -- one of the nicest mounts in the game -- requires a druid with epic flying to summon the required boss, which seems steep enough (I've offered to summon for anyone in my guild if they have a proper group, with the added bonus of my passing if it drops). But people often forget that everyone in the group needs Honored with Lower City as well. When you only need a reliable tank or healer, this requirement is disastrous. Even in BC days, people skipped Terrokar Forest and ignored Lower City. Now, even with all the DK tanks, it's impossible to find one with the required reputation.
The best way to remedy this horrible mess is by being a tank or healer -- if you're lucky, the druid you bring will spec for the other role (my Plum does heals). But if the only people going for this are dps, you will get royally screwed. Often.
There was only one tank online this morning when two rogues and I were set to do Sethekk, and we forgot to ask about his rep. And, of course, it was too low. One of the rogues suggested a Shadow Labs run, but I bowed out since I'm trying to hit 80 on Dusty today (so I can do heroics with family tomorrow and because everyone wants a healer these days). While I'm fully willing to interrupt my regularly scheduled playtime to summon Anzu and pass on the mount for a guildmate, I'm not quite masochistic enough to help farm rep.
I think, and correct me if I'm wrong, these are the only mounts you have to "attune" to farm anymore. Nice mounts, though, and worth the effort if you can find a regular tank and heals.