A chronological list of everything directly connected with contributions in one's profile page. Whenever I log in, I want to see all responses to my arguments, rebuttals etc. as well as new debate/popularity topics. Currently, I have to click the 'New argument/debate activity!' which seems to demand a lot of brain RAM.
Also, ditch the weird animated (AJAX?) login box. For some reason, I have to re-enter my credentials every visit despite telling Firefox (and ticking the form checkbox) to remember me.