Spicytuna has a nice little writeup about a much-discussed but never implemented feature in World of Warcraft: player housing. Blizzard has borrowed (and subsequently improved upon) many of the most common features in MMOs -- they revamped leveling with ideas like rested XP and recruit-a-friend, they changed the endgame with the ideas of Heroic instances and daily quests, and they've tweaked PvP with battlegrounds, Wintergrasp and Arenas.
But for some reason they've never taken on the idea of player housing: a place in the game for players to make their own. The reason we've always heard is that they never landed on a good implementation of it -- if they couldn't do it right, they wouldn't do it at all.
But Spicytuna proves there's no shortage of ideas. The main thought so far is that such an area would be instanced, as having actual buildings in the game as player houses just leads to emptied out ghettos of buildings left to rot.