Automatic tag generation?

Would it make sense to automatically tag posts by their content?

Indeed it does, which is why metadata scraping using Embedly's "Extract" API is already in place and being experimented with and Tag autocomplete is already in development as well.

p.s. Anything that can make the submitting and organizing of the site easier and less work for people, including automating as much of the process as possible, is a good idea IMO.