I just realized the other day that Blizzard brought back attunements in Mists of Pandaria. Only, in classic Blizzard fashion, they flipped them and made them driven by rewards, rather than by punishments.
The purpose of attunements is to provide a guide through content. Something that tells the player if she is ready for new content. In The Burning Crusade, that role was played by attunements. You couldn't go to Kara until you had done the quest chain, same with T5, and T6 required defeating the bosses in T5.
Now, you can jump ahead if you want, but if you follow the Legendary questline, it paces you through the content. First you do some 5-man dungeons, then you do Highmaul. You repeat Highmaul for a few weeks, and gear up as a side-effect. By the time you've done the Legendary stages of Highmaul, you're ready for Blackrock Foundry, and the process repeats.
The reward, a Legendary item, is a very strong motivator. Pretty much every guide says to start the Legendary quest line as soon as you can. That encourages you to stick to the quest line, which matches you to content that you should be geared for.
The modern Legendaries occupy the same design space as attunements. But rather than restrictions on players, they are seen as rewards. The carrot, not the stick. Players also have the freedom to skip ahead if they know what they are doing, and there are no issues with raid composition that previous attunements used to have.
I'm kind of bemused that I didn't see this before. It's excellent design work from Blizzard. And what's most amusing and impressive is that it's the same trick Blizzard always pulls.