Here's an interesting observation: after patch 2.4 drops, Onyxia will be the hardest attunement in the game. BT and Hyjal dropped, SSC and TK dropped, and even the Karazhan attunement got removed, so the hardest attunement now left in the game (does the AQ gate opening count as an attunement?) will be the long quest chain to attune to Onyxia's Lair.
And Blizzard is apparently OK with that. They've been back and forth on attunements since the beginning of the game. On the one hand, attuning a character to an instance like Onyxia makes things feel epic (and indeed, some people think the Alliance version, with the Stormwind reveal, is one of the best quests in the game). But putting attunements in the way just to keep players away from content clearly isn't the way to go about things.
It will be interesting, as always, to see how things change in the next expansion -- Blizzard has already said that they'd like alts to be able to benefit from attunements and reputation rewards, so odds are that while there still will be attunements of some kind, they'll be set up in such a way that once you do them one time, you won't have to do them again. And that's a pretty good balance -- players still get to experience epic content, but it never is forced into a grind where you have to jump through hoops just to get someplace the devs don't want you to be yet.