Productivity is important in research. Ultimately, you'll be judged on the importance, quality (and number) of papers that you publish. This got me thinking about whether there was a simple way of encapsulating this and I came up with the following:
Aim to do more and more important research per unit-time.
It's sort of obvious, but the key quantity is the rate at which you produce important, high-quality research. If you hardly ever publish anything, that's a bad thing. If you publish plenty, but all your papers are low-grade rubbish, that's a bad thing too. And even if you produce large numbers of high (technical) quality papers, but they're all studying unimportant problems, that's not great.