Update: This will not affect quest experience.
This isn't very surprising, but Blizzard wants you to move on to new expansions when you reach a certain level. And today Nethaera revealed a fairly significant alteration to the way players gain experience, based on your level and the content you're doing at the time.
This only affects NPCs that you kill; quest experience rewards will remain the same. Essentially, once you reach the maximum level for an expansion, all of the creatures involved in that expansion will only reward 10% of their normal experience values. So, at level 60 you'll need to move on to Burning Crusade content, level 70 for Wrath, and level 80 for Cataclysm.
Level 60 or higher players get 10% normal experience for killing creatures in Azeroth (“Old world” content)
Level 70 or higher players get 10% of normal experience for killing Burning Crusade creatures.
Level 80 or higher players get 10% of normal experience for killing Lich King creatures.
Level 85 or higher players get 10% of normal experience for killing Cataclysm creatures.
It sounds like this is in response to players grinding out additional levels in the older areas -- for one reason or another -- instead of moving on. Nethaera mentions the "ease" of content being a major factor in players sticking around after they've outgrown a zone. But then again, they're also missing out on a ton of great upgrades from quest rewards -- especially new characters -- so it's not exactly advantageous to keep grinding away in old areas anyway.
The change is a strange one, but maybe I'm just missing something that happened recently to bring it about.