I have my entire Dreadnought set, two Misplaced Servo Arms, Ashkandi, a full SR set, a full FrR set, a full NR set, a full FR set, T4, T5, quite a few pieces of now-outdated PvP/Kara/SSC/TK gear, 3013 Apexis Shards (soon to be potions!), every 2H weapon from Kara through BT (only missing Kael and Archimonde's sword since I've passed it to people who will get a bit more use out of it) and probably some other things I'm forgetting.
the old resistance gear is crap compared to the new gear they have now, the only few items worth mentioning is gaea's embrace and the FrR cloak [name escapes me]
I still have a bunch of old stuff. I've got everything but the legs from my Valor set (well, some parts are now Heroism), because I intend to go back and finish as much of the upgrade as I can (or maybe the whole thing) someday. I've got my Ornate Adamantium BP, just because it was such an awesome tank piece for the time. Ditto my legs and boots from the long quest in Silithus that you had to have a raid to finish (had to summon the Egyptian-looking guy). I have the Silent Fang I tanked with for a while, because of the proc - that silence proc saved me more than once.
Stuff like that... Just sentimental stuff, mostly.
Well this game should be about fun, not owning or killing some virtual dragons. (almost) It is most funny.. when you make arcanite reapers with someone and duel each other having the S2 set or so. As I have seen in one PvP video.
You can keep them just for fun.. if you have the space in bank that is.
Honestly, when I dinged 65, at the point most of the level 60 epics I had faded in effectiveness, the vendor-hammer struck hard.
Onslaught Girdle, Lionheart Helm, Onyxia Tooth Pendant, Titanic Leggings, Savage Gladiator Chain, ZG bracers... VENDORED! The rest of my gear was rank 14 pvp stuff, so I kept that. Doesn't vendor for anything, no sense in destroying it.
Sometimes I want to do nothing but old-world raids. For that old nostalgic feeling. :]
If you were a Rogue, you'd still have your lvl 10 Certificate of Thievery. And you'd never know if you could get rid of it or not, so it would sit there, in your bank, just occupying space.