Allowing a user to edit their posts wouldn’t be hard, technically. I’m kind of thinking that I like the current setup though, where you can delete, but not edit. It encourages you to not put too much into a single post. It also fits with the “publish” nature of the site. Once you’ve hit “submit”, it’s kind of like sending an email; anyone else could get/read it immediately. So you should think before you submit instead of after.