You've probably seen the news that Eve Online is only 4% female. Naturally the blogosphere is brimming with reasons why this is so. In my mind, there's probably no one reason that contributes to the lack. There's probably several reasons, each chipping away at the potential audience.
For example, if the ceiling for female participation in an MMO is 40%, maybe Eve loses 5% because it's hard scifi instead of fantasy, 5% because it emphasizes PvP, 10% because of the horrible reputation of its players, 5% because it's boring, and so on. (All numbers made up, just illustrating that a series of filters can reduce the audience greatly, even though no one reason is fully to blame.)
Another possible reason is the different cultures that play Eve. For example, a large number of Russians play Eve. Maybe much fewer Russian women play video games as compared to North American women. This imbalance might knock another few percent off.
However, I would like to discuss one potential reason that I haven't seen anywhere else:
Eve Online is the only major MMO without a humanoid avatar.
At least, an avatar that actually makes a difference in gameplay. All you really have is a portrait and an avatar that you can walk around with in the station. In Eve, your "real" avatar is your current ship. Even Star Trek Online has your captain as an avatar, and using your captain makes up a good half of the game.
There are a number of research indications that, in normal MMOs, women identify more with their avatars than men do. For example, less than 10% of women will play with a male avatar. In comparison, 30-40% of men play with female avatars. It's possible that one of main reasons that women avoid Eve is that there is no avatar for them to identify with, just a ship. Maybe many women would rather play a character, rather than play a spaceship.
If you accept this idea as possible or true, it leads to an interesting perspective on the Incarna debacle of 2011. In Incarna, Eve introduced 3D avatars, as well as micro-transactions to outfit those avatars. The playerbase, lead by the CSM, revolted, mostly because they saw the avatars as a waste of time in their spaceship game, and because they thought Eve was getting greedy with $100 monocles or whatever. I think Eve ended up dropping the microtransactions, and didn't really do anything with the avatars, though they were left in.
But perhaps one of the reasons for Incarna was a subtle attempt to raise the proportion of women in Eve by providing an avatar that could be identified with. An attempt that wouldn't really affect the rest of Eve proper, the way attacking any of the other filters would have. Of course, the micro-transaction side of Incarna put an end to continuing this experiment in any meaningful way.