[Rythonn]

UO

Smelting - for the love of god why can't we smelt scrap armor back into ingots, even at a loss.
Housing - UO had the best housing at the end, build your own anyway you want.
Vendors - Being able to place vendors at your house or to create some sort of shop.
Karma - I loved the UO pvp system of murderers and flagging for pvp.
Runes - being able to mark a rune for a recall or moongate was great. Granted wouldn't probably work in WoW world isn't big enough.

SWG

Tip - every game needs a /tip command.
Resources - Best of any game having resources vary in stats and combining the rare stuff would get you the ultimate armor/wep but was costly.

EQ

Teleports - kill the gryphons and lets go back to a teleport every 5 minutes. I would rather sit on the ground doing something useful waiting for my ride then traveling on some winged beast I can't control and cant stop or change direction.