Some of us already use metadata in our daily coding - for example commit messages.

Recently I've noticed, that I comment less, and instead write more information in commit messages, and do svn blame more often.

It's better, because it has more context (you see to which changes this comment is relevant), and is always up-to-date, because you always write fresh commit messages.

We have a rule in company, that you have to add task number from JIRA to each commit message, and this alone is a huge improvement - you can always check WHY some change was made).

not necessarily, you could do what docco does and assume that comments pertain to the lines of code immediately below them, up until the next comment or the end of the function

Well, we could use a commented syntax and build editor support for it. If HTML, <!-- COMMENT: lorem ipsum -->. If C/Java/etc. then the same but with /* */.