Automatic titling was recently enabled for internal link to questions, to lower the number of plain links with no descriptive text. I think this is a great change, and think it would be great to extend this to external links.
When a plain link is posted, the URL could be fetched by the system, and if there is a
<title> attribute on that page, the link text could be changed to that text. To prevent absurdly long link text, it could be truncated to, say, 100 characters. If the link were just the domain (e.g.
http://stackoverflow.com/) or maybe just the domain and a single subdirectory (e.g.
http://stackoverflow.com/faq/), it could be allowed to remain since that type of URL is fairly clean and may sometimes be useful. If a user absolutely needs to have the link text be the URL, then they can do so manually.
I understand that there would likely be some challenges with implementing this. It is not however impossible - Facebook does something similar every time they fetch a page that a user links on their wall, and other sites surely do similar such things. I think this would be very helpful in lowering the incidence of bare-bones links to at least bare-bones links with titles, and a descriptive title goes a long way.