Wouldn't it make sense to have this related to the number of games you're playing. I know other sites that do something similar and I think it's a great idea. The fewer games you're playing, the more quickly your rating moves. And vice versa. This way, if you're playing 200 games, you rating won't oscillate hundreds of points when you lose a couple. And if you're only playing six, you won't have to wait forever to get your rating up. I know people like to see big rating changes so you get more satisfaction from a win. But it's not fair for your opponents that are about to lose and have to agonize over your 300 point rating drop because you weren't keeping up, or just had a bad streak.
I'm only playing 40 games, and I'd like to see a lower K factor for me. I can't imagine what it's like for those with 200+ games. What ends up happening is their rating doesn't accurately represent their ability.
It actually might make more sense to have K be determined by your moves per month, but that involves a more difficult calculation.