Can you clear it?
Just curious. I don't need to but was just wondering. The less info stored about me the better, on any website
Clears after 30 days?

Keeps only a limited number of threads, so if you were viewing a lot in one day it doesn't even keep more than the last few hours. Could make it wipe sooner, but I think most people will see the functionality as a greater benefit than privacy concerns given that nothing is leaving the site.
I share same privacy issues in general, but....It doesn't track you off the site. And it doesn't store anything anywhere else. Your browser history, sent/logged by/over your ISP lines, is more susceptible to review and/or man in the middle attacks. For those that don't wipe their cache one every browser close (99%+) there is BY FAR less information here than is stored on their ISP or home computer.