you don't need links any more, you need advertising and a great user experience
It can only solve half the riddle. Presently, reach of Google in getting reliable data on user metrics is limited. The challenge only increases for smaller sites with lesser traffic. Which is why, Panda mostly affected larger sites. But, going forward Google's reach may improve and so its leaning upon user metrics.
User metrics, in isolation, may not be the answer to everything. There may be a site like answer.com that people spend only a few seconds on, mostly on a single page looking up for a word.
Besides, user metrics will depend on the relevancy of traffic. How is Google to know what keywords to rank a website high on, if it doesn't know what the website is about? It might be a great site, but you send it the wrong traffic, metrics look pretty ordinary.