One of the more fashionable business concepts of the early 21st Century has been the long tail. This has taken off because organisations like Amazon are able to hold a much larger quantity of stock than conventional retailers, so are able to cater for a wide variety of minority tastes that a high-street shop could never hope to match. They can do this because their business model locates the product storage in cheap out-of-town depots, so the cost per item is much lower. In principle, all-digital stores (such as iTunes) should be able to cater for an even wider range of tastes, as the cost of storage and delivery for purely digital goods is even lower.
MMO makers are in a similar position. The cost of creating and storing new content is very low, so games like WOW can afford to include features that appeal to minority tastes, such as role-players or hardcore raiders. If someone sits on a virtual chair or kills an Internet dragon, they don't wear out or need to be re-made. The only thing that limits their re-use is the boredom of the players; a thousand people can sit on that chair simultaneously, but the same player won't want to sit on it a thousand times.
One of the things that makes life difficult for new entrants to such a market is the ability of large incumbents to spread the (already low) cost of new features over a wide player base. One of the interesting things about Rift is how the developers appear to have addressed this. They can't match WOW for player numbers to spread costs over, but there is another way. By developing tools that allow them to create content more efficiently than Blizzard can (as demonstrated by the rapid release of new content in patch 1.1), they can compensate for lower initial subscriber numbers and hence add in enough to attract a wider audience over time. It's a tactic that Blizzard will find hard to counter: re-working the ageing WOW engine to allow for rapid content creation is a significant undertaking, particularly when the top developers are away working on the mysterious Titan project.
Titan may well be the plan, of course. Whilst it would be hard to port WOW to a new, more efficient engine, it's far from impossible. Whether it's cost-effective is another matter. Given that player boredom kills everything eventually, it may well be simpler to let WOW gradually fade and move on. After all, it would still be profitable if it lost 90% of its subscribers. That's a real long tail.