Arnon Rotem-Gal-Oz has discovered requirements traceability in Mingle, which turns out to mean, "If you mention a card number [agilese for 'ticket'] in your check-in comment, Mingle automatically converts the reference to a hyperlink." Trac, DrProject, and several other systems have had this for years; I think it's too weak to deserve the label "traceability", but it's still a nice feature if people are disciplined enough to use it.
And a non-link: I was talking with Carolyn MacLeod and Rick Salay yesterday about the benefits (if any) of high-level modeling using things like UML. Rick made the point that pre-code models allow you to think about design issues before being distracted by details like the number and types of parameters to particular methods. We decided that premature concretization is the real root of all evil (in software, at least). Problem is, "concretization" is a really ugly word. If you can suggest a better one, please do.