One of the biggest headaches any wordpress user will have is dealing with spam comments and user registrations. I manage a few wordpress sites and I try to install recaptcha on every one I use and it seems to work the best.
I really think it would be a smart move on wordpress's part if they made this feature optional from the basic setup. It should even ask you while you're installing if you want to enable captcha.
I think this would take away a lot of headaches for wordpress users and encourage more people to stick with wordpress.