There are lots of metrics to track in startups... most are what Eric Ries calls "vanity metrics". While you do need to focus on real, actionable metrics, I take it a step further.

What's the one number you have to focus on for your startup to success at this very moment? Sometimes it's revenue, sometimes it downloads; we focused on signups.

Everything you do should drive growth of that one number - otherwise you shouldn't do it.