I don't think the open ended aspect of a single player game can be compared to a MMORPG, the latter has a far bigger potential for it, say scale discrepancy. One of the comments of the article you posted zyrmugeist said it all, talking about WoW's expansions. WoWish games have a clear solid goal at least for me: get all pve/pvp gear sets, then you just finished it, you can do battlegrounds, arena and anything else but the carrot-on-a-stick get phat lewt structure ended. That's why expansions are necessary - because rolling an alt is like playing the game exactly like before but you get different tools for killing, same quest grind (or maybe mob grind if you're masochist) until you can finally reach the max level, in order to get the best equipment possible, then you finished it again.
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I don't think the open ended aspect of a single player game can be compared to a MMORPG, the latter has a far bigger potential for it, say scale discrepancy. One of the comments of the article you posted zyrmugeist said it all, talking about WoW's expansions. WoWish games have a clear solid goal at least for me: get all pve/pvp gear sets, then you just finished it, you can do battlegrounds, arena and anything else but the carrot-on-a-stick get phat lewt structure ended. That's why expansions are necessary - because rolling an alt is like playing the game exactly like before but you get different tools for killing, same quest grind (or maybe mob grind if you're masochist) until you can finally reach the max level, in order to get the best equipment possible, then you finished it again.