Challenge Modes should be top priority if you've got your eyes on those Mists of Pandaria sets, because they will no longer be available as of 6.0.2 -- the phoenix mounts will also be going away, although players that have earned one of the mounts will have access to all variations. Players looking for heirloom weapons from Garrosh Hellscream should hop to it, as those will also be removed once Warlords of Draenor is released in November. In addition, the Brawler's Guild will be taking a hiatus once patch 6.0.2 hits -- the bosses and encounters in the Brawler's Guild will be re-tuned for level 100. However, if you've already gained access to the Brawler's Guild, you'll still retain that access when the new expansion launches -- no need to dig up another invite.
Lastly, if you have not started the legendary cloak quest chain by patch 6.0.2, you'll no longer be able to obtain it. Players already on the chain will still retain the quests, although they will be slightly altered, as the section of the chain that requires players to collect Valor Points will be gone altogether with the removal of Valor Points in 6.0.2. For more information on changes and removals with the arrival of both patch 6.0.2 and Warlords of Draenor itself, take a look at Blizzard's full post on the official site.