BlizzPlanet interview with Tom Chilton last night.
Q: Ghostcrawler hinted at an unannounced feature in this expansion that is going to really change how WoW is going to be played, much like the LFR feature – can we get a hint on when we might see this feature and what it might pertain to?
A: We will probably see it in the 5.4 neighborhood if we are able to get it done in time. As far as hints a lot of players in the past have suggested that they be able to do older content so it’s probably something that goes along those lines.
So there we have it. Something to do with older content.
Old Raids added in a meaningful way to LFR, possibly with up/downscaling involved.
Rewards: Some sort of 'Legacy Points', used to buy any old items for transmogrification.