Familiarity with functional/declarative programming methods. I'm not a guru on this at all but learning a different way of thinking about programming like this

Familiarity with functional/declarative programming methods. I'm not a guru on this at all but learning a different way of thinking about programming like this has been terrifically helpful -- in some languages more than others. You can write functionally to at least some extent in almost any language -- even Perl has a level of support for it that surprises a lot of hackers. So if you haven't already, learn a Lisp, or Haskell, or even JavaScript. Something that lets you pass around functions as parameters, and does closures. It's superb for JavaScript, for example, once you realise it's basically a slightly weird Scheme with C-ish syntax. Suddenly all that ranting about closures and lazy evaluation will seem so much more relevant!