Came across this thread on inbound.org about the "Top 3 metrics every content marketer should track". Had to weigh in after reading the responses because, well, they catered to the wannabe blogger superstar and had very little actionable insight to measurable business goals.
Since we're talking about content marketers, whose role is presumably to drive awareness, leads, and retention through the stuff they help put out online, I'd say the most important metrics circle around those three items.
Specifically, I'd look at:
Time between buying stages (if I'm doing my job, then it's getting easier to sell my product/service, and we're controlling more of the buying discussion because our customers are more educated by the time they reach us)
Half-life of content promotion via email, social, and paid ads (if I'm doing my job, then I'm building momentum across all three of those channels, sequentially, and it will take longer for that momentum to die down).
Customer Lifetime Value (if I'm doing my job, then our customers are finding more ways to be successful with our product/service, which means they're staying around longer and buying more from us).
Everything else is just inflating your self worth.