I did this a little bit earlier, very politely asking them to delete my account and why I wasn't happy with the direction they went with the site. I haven't yet heard back from them yet, but it's probably a slow process.
I don't like that they'd automatically opt me into displaying what I read. That ruins any sense of privacy I have to browse the site (ignoring the cynical and probably accurate view that I had no privacy there to start). Were I curious about something but didn't want it shared with the world, Quora is a dead end. I can opt out, but if this is their idea of the future, they may remove the option.
In the meantime, I tried deleting most of my answers, though Quora doesn't really do so. Instead, changes are kept in a list of edits, which allows you to revert the change. I like that, it's handy, but I can't remove my edit history. My hope is that the changes are lost once the account is deleted, though I doubt they will.
I also dislike that deleting my account, including all data associated with it, requires I e-mail them. It seems absurd to do all this work for your users and provide them no way to easily walk out if they're unhappy with site changes (as I am). If I own the content I publish on Quora, as they claim, then I should be able to completely delete it.
At best, it's a way to prevent accidental account loss. At worst, it's a slimy method of ensuring the service holds onto user-provided data. Sadly, the only site to allow me to delete my account without sending an email thus far was Facebook[1] -- though I'm sure they've held onto a great deal of my junk nonetheless. At least they provided the option, I guess.
[1]: I believe Google also allows this, though I haven't quite gotten to the point of deleting my Google account.