As users grow, QA sites are very likely to come up with junk posts. Two improvement suggests:
some newly asked questions actually already have been answered, it's better for registered user to redirect it to the answered question;
if too many users post junk contents as ads, etc. or if you want to improve the quality of questions and answers, why not shut down public registration, only open sign-up function to the ones who worth it, by invitation sent from registered users.
hope those will come true~