Massively

Free expansions:
The standard MMO development model involves charging for periodic expansions with new areas and content. EVE Online has taken an alternative approach that has been a massive part of its success. Nathan "Oveur" Richardsson, CCP's Executive Producer, said in an interview that he considers the cost of expansions to be something that's included in the subscription fee. EVE's expansions are therefore released for free and applied to the entire game world. This may actually pose less of a headache for the developers as they no longer need to support old content that is changed with an expansion and there's no need to separate players into different areas based on what expansions they have. In addition, the idea of getting free expansions helps to gain extra subscriptions and the money made over time from those subscriptions easily outweighs the money they'd make by charging for expansions.

Iterative development:
While most MMOs will release free balance and bug-fix patches throughout the year, very few add all of their new content for free. EVE Online does, and it's all part of a long-term iterative development strategy in which the game is a constantly evolving product. Rather than seeing the game as a set of individual sales, subscriptions are seen as upkeep for continued development costs. The effect of the game's constant evolution is that it keeps players interested and helps prevent the game getting stale. As a result, EVE's player retention rates are significantly higher than average, which leads to a constant gradual increase in subscriber numbers. Players still come into the game at the same rate but they tend to leave at a much slower rate.

This iterative development strategy isn't something that's restricted to just EVE Online. In one form or another, it has been the cornerstone of several other MMOs that all exhibit the same gradual subscriber growth. Lord of the Rings Online updates (called "books") often contain new content and Runescape, which is popular among younger gamers, releases new quests and areas to the game every few weeks. Players tend to see their MMO subscriptions as a long-term deal and seeing that they're getting something new for their money every month makes the deal a lot more palatable and makes them less likely to quit.

Server model:
I went into some detail on this in a previous feature but this article wouldn't be complete without mentioning EVE's server model. CCP made the decision early in the game's development to create one massive server where all players would interact rather than creating multiple servers or shards where separate instances of the game world exist. This has had far-reaching consequences for gameplay, making territorial conflicts feel more real than they would if other servers existed and forging economic stability in the player-run markets.