In a timely fashion the news of today develops my point from the previous post a bit further: If we assume a market full of MMORPGs with relatively similar gameplay and mechanics, the competition in that market would presumably be about content. Just like competing TV channels, the question would become who offers the most engaging content at any given time. This is one area where Blizzard appears vulnerable, as for most players the amount of content offered in any one expansion plus related patches is not sufficient to fill up all of the two years Blizzard needs to bring out the next expansion.
On the other hand, any minor content addition to World of Warcraft automatically becomes big news. Thus today's headline on many MMORPG sites is not "Rift head-start launches", but "WoW brings back Zul'aman and Zul'gurub". With the conspiracy theorists of course claiming that this is deliberate timing by Blizzard.
Whether that is true or not, just the fact that some people can believe that shows us how far ahead in the content competition game World of Warcraft is. On any reasonable scale, the release of a whole new game with lots of zones and features would rank far ahead of an announcement that in some time in the future a 6-year old game will recycle two old raid dungeons as 5-man heroics. But no, most players are far more excited by another chance of getting their 17th "rare" mount than by a new game launching.
I am not a psychologist, but I would guess that this is coming from something like an endowment effect for virtual goods. Not only do you value the character you already have more than a similar character you could create in a new world; but also the "rare mount" in the game you are already playing is perceived to be more valuable than a similar "rare mount" in a new game.
I like trolls, at least in WoW, not so much on blogs, so I'm looking forward to playing Zul'Aman and Zul'Gurub again, albeit hopefully shortened significantly, or split into wings. But as long as "WoW adds a third and fourth recyled old heroic dungeon to the game" remains big news, other games will have a hard time getting anywhere in the content competition.