I write MLP fan-fiction under the name Bad Horse. I'll skip the reasons why I write fan-fiction. There are a few reasons I chose to write ponyfiction as opposed to other types:

• There are only a few large, active fandoms: Harry Potter, Twilight, Avatar, Naruto, and My Little Pony.
• The ponyfiction websites are much better-designed than those for the other fandoms. There are no fanfic websites for any other fandom that have a ratings system, so it's too much work to find anything good to read. And I didn't want to just write and not be a member of the community. Even if I were arrogant enough to do that, it wouldn't work very well even from a completely Slytherin viewpoint. This is the most-important factor.
• You can write anything as ponyfic. If you want to write a story about an old woman whose children have left and who has refocused her attention on her flower garden, and a travelling salesman who figures this out and manipulates her love of flowers to sell her something, and you wanted to do it as a Harry Potter fanfic, you'd have to make her use spells to grow her garden, or something stupid like that. I can't even imagine how you'd do that as a Twilight or Naruto fanfic. But anything becomes a ponyfic when you make the characters ponies. This is probably why I like My Little Pony. Harry Potter is about the wizarding world. Avatar is about a world war. MLP is about people.

The short version: the show is good and sincere (but trying it in the first place requires high openness), a large subset of LW's current readers are into fanfiction, and the brony community in general creates lots of fan materials.

The 4th generation of the show started 2 years ago, and is very well-made, is happy, and less targeted at little girls than the previous versions. Generally, boys shows got more talent and funding because network execs knew that both boys and girls would watch boy shows but only girls would watch girl shows- which was, in part, a self-fulfilling prophecy. Why watch technically inferior shows? (Most male fans of the current generation find the design of previous generations hideous; I don't know how the female fans feel about it.) The show still has a definite feminine feel- the protagonists are all female, and the majority of the episodes center around personal interactions and friendship problems- but there are also "save the world" adventure episodes, and a strong diversity among the characters in terms of both skill and personality. The personalities are iconic and basic enough that most fans identify strongly with at least one of the characters.

I only gave the show a try, though, when I needed to watch television and had watched every other show on Netflix made in the last five years that I expected to enjoy. (I was only recommended it once or twice.) Hearing that a show for little girls is actually good and giving that hypothesis enough credence to watch the show seems like something that requires high openness- which is a trait I suspect is associated with being an LWer. (When much of your social circle approves of it, the amount of openness required is obviously lower.)

One of the things about the show that's polarizing is that it's very sincere. Ponies are, in general, happy and like each other. Compare to, say, Watchmen or The Wire. One way to show sophistication in modern society is by appreciation of irony, but LWers strke me as more likely to construct their identities out of sincere enjoyment rather than ironic enjoyment. The sentiment that, really, 2+2=4 and, really, capitalism creates wealth seem to resonate here much more than they do in some other places. Somewhat related, it seems possible that people who are less socially adept are more likely to become bronies, but I don't have enough details there to more than speculate.

As for writing fanfictions, I found LW through HPMoR; when I heard "here's some Harry Potter fanfiction, and it's really good" my response was "okay, I'll give it a read" not "fanfiction? And for a children's book?" Similarly, once I started watching MLP I started consuming a lot of MLP fanfiction- and so after a while the thought of "hey, why not write my own fanfiction?" popped up more and more frequently until I put pen to paper.

There is an entire subculture called 'bronies' of adult fans of the show. My best guess is that the demographics of LessWrong readers come closer than average to the demographics of bronies, and so there is a higher-than-average chance of a LessWronger being willing to be a brony.

As for why the show is popular among adults; the best theory I've heard is that unlike many modern cartoons, it's unashamedly non-cynical, and is actually reasonably-decently written and produced, which makes it near-unique at the moment.

Funny you should say that. I'm currently about a third of the way through writing an anti-death fic written as a reaction to Eternal and some similar stories. Let me know when I can read yours; I'm quite curious to see how you approached it.

It hasn't been posted yet, but I am looking for alpha readers at the moment. Send me a PM if you're interested. I don't work at anywhere near 2k words a day (well, on this at least- I have other nonfiction writing projects going on), and so I don't expect it'll be as long as yours or update as frequently.

I'm made of sterner stuff than that! And if they're not, well, sucks to be them :P

So far, the premise of mine and the premise of DataPacRat's seem very different, and so it's not clear how rivalrous comparisons will be. (I've only read the first chapter of theirs so far.)