Wondering why users do what they do? Because you designed your site that way
drawar.com
drawar.com
I might have something valuable to say but I don't bother jumping through hoops, ever. I visit dozens of sites a day from hacker news and I'm not signing up for a dozen blogs a day. The cost is just way too high. Instead, I'll just comment here.
Admittedly, requiring people to sign up will stop people from posting truly contentless replies. But there some very intelligent well spoken people who just won't bother -- they have better things to do.
Certainly, what users want to do is influenced heavily by what you want them to do. Good design sets those expectations well.