http://justdelete.me/ is a good site for looking up these policies. Skype is bad, but not quite in the "Impossible" tier with sites like YouTube, Wikipedia, and Hacker News.
According to justdelete.me, "If you haven't posted on the site, it's just one click. Otherwise, edit your 'About Me' bio to say 'please delete me' then contact support."
On SE, you can delete your identifiable information, turning your username into one of the generic "userXXXXXX" names. All of the content you've produced remains, though.
Which is really great. I hate looking for some obscure problem, find where someone posted a fix online, then discover the account of the fixer was deleted and the fix is gone now. Even worse when it's a program or file or something...