Wow, so many great responses. Thanks to all. I'm looking forward to digging in and studying this stuff.
Alexander, I met a YC alum and the impression I got is that Paul Graham suggests having a range of metrics and focusing on one per week. Traffic and revenue are obvious, but chances are most startups can't grow revenue 100% in 10 weeks.
But there are tons of metrics: social shares, time spent on the site, customer reviews, etc. Most most startups can pretty easily say, "We'll get 10% more X" in a week. Apparently just having small goals like that creates momentum that ends up boosting bigger picture metrics, too.