I have a full Magister's set I farmed in WotLK, none of the replica BS. Also got the Fiery Warhorse back in WotLK too on some random 4am Kara run. Have a bunch of random crap in my bank too that doesn't exist anymore but nothing truly worthwhile.
Light of Elune. Something tells me it's rare than 90% of things mentioned in this thread.
This was definitely the first thing I thought of. I've got a few neat items and mounts that are no longer available, but Light of Elune is both no longer available and consumable, so even if you had it at one point, most people have probably used it. I'm still waiting for that one special moment when it seems worth using it after all these years.
Swift Razzashi Raptor
Ashes of Alar
Runesword of the Red (Weapon from ringing AQ gates)
Grand Spellstone (old Warlock conjured item. Still got it in my bank)
Scepter of the Shifting Sands
Captain Sander's Shirt (discontinued)
Sawbones Shirt (discontinued)
Thug Shirt (discontinued)