This week's issue of the Escapist has an excellent article by Greg Tito discussing the difficulties of being a mod writer in World of Warcraft. And we're not just talking about learning LUA or fixing bugs after Blizzard has released a major patch, but about the community itself. You've all read the official forums, so you know the community isn't always friendly. And when a mod you've written goes from being something you coded for yourself (and uploaded on a whim) to something used by thousands of people (each of them asking -- or demanding -- different bug fixes and improvements), I'd say things stop being fun. Mod-writer Gello explains some of the stress of the situation when he tried to help his users out by writing a German localization:

I had spent a couple very intense months working on the localization of Recap. It got so I could understand combat logs in German.... It was just causing way too much stress and time for something I would never see or use.

And when French players began requesting a French localization of mage water-summoning mod WaterBoy, Gello refused, suggesting that native speakers could make a localization themselves. And then, as Gello says, the flames began:

When I stood by my position (probably not in the nicest terms), they continued in earnest. I got an email with an attachment I thought was safe and apparently it wasn't.... I basically abandoned the email address, formatted my pc, ditched the mod and didn't look back.

Really, people: go give your favorite mod writer a hug. It can be an immense amount of work to maintain a good mod and most of it is thankless.