What's the point of spending 4+ years building a large scale MMO if you can't hold onto your players? With so many MMOs to choose from these days, the most important design factor should be the retention rate. There are many things that contribute to the retention rate of an MMO, but the biggest by far would be the game play itself.
In the past different game companies have tackled retaining players with different strategies. Blizzard used an insanely long leveling system as well as a bunch of really hard end game content. Their strategy was to always give players a goal upon logging off. There would always be that slightly better item, or that incomplete challenge that would leave the gamer unsatisfied and needing return. Ever wondered why in WoW high level players were allowed to run around and gank lower level ones? It would encourage the lower level ones to level more and seek either revenge or protection from it ever happening again. Blizzard's strategy has definitely been the most successful to date, however I can't see it working particularly well in the future. Gamers are waking up to the fact that endless grinding for bare minimum reward is a waste of time.
New retention strategies will be coming into play and I think they all revolve around convenience. The ability to log on and do what ever it is you want to do in the game with bare minimum effort is the key! There are a lot of people with very little spare time, who want to get into the MMO scene but can't invest the time. One issue with making a game convenient is balance. You can't make certain aspects of the game more convenient than others, or else everyone will flock to that. WAR is a prime example of how convenience imbalance can cause major problems. The majority of people only play scenarios for the leveling convenience. I recently read a brilliant article over at NecroRogIcon explaining why convenience is the trump card on all other aspects of game design. Definitely worth a read.
Some other contributing factors to gamer retention are pricing models, rapid game patching, customer service and of course a fun game with copious amount of content. Using a subscription based pricing scheme is risky. If the game launches with a bunch of problems, players are going to cancel their subscriptions within the first month. Let's face it, which MMOs don't launch with problems? Snafzg had a great idea about extending the initial subscription period included with the purchase of the MMO to 90 days instead of 30. I thought this was a brilliant idea, as the more time a player invests into a game, the less they want to leave. I think moving forward, a non subscription based pricing model will have a better success with retention rates. It allows players to change between MMOs with minimum hassle. Very appealing to the casual gamer, just look at Guild Wars.
Rapid and responsive game patching is another key element to keeping the crowds. If a player has a problem with the game, but can see it being fixed in the near future, it will help to keep them from leaving. Good customer service also goes a long way.
The MMO game industry is still young and lessons are being learned every day. New MMOs are popping up all over the place and old ones are hanging around. With so much to choose from, the gamer needs a reason to stay and it can't be the endless grind factor anymore.