Wrath of the Lich King Heirlooms
The experience bonus will stop working. It's also possible they will no longer grow in stats above level 80, but you can probably still equip them.
And I just wanted to clarify. Did that mean that you will no longer get an exp bonus from WOLK Heirlooms at all? Or they just won't give the growing stats and the exp bonus from 80-85? I have a bunch saved for a Hunter I want to level in Cata, so I hope I can still use them.