The signal will be hugely susceptible to outlier bias -- The Post That Goes Viral, and generates a huge number of incoming links -- will dominate the rankings.
Because the signal is so thin, distorting and manipulation (through link farming) will be cheap and difficult to detect (a small number of links across a large number of sites).
Don't get me wrong: looking at incidental behaviour is useful, and can often be much more beneficial than direct actions. But remember that all of these signals are actually proxies for some ineffible quantity you're trying to measure, quality.
(The very definition of which should leave you crying on the floor after a few hours. Or days. Or weeks. Or months. Or years....)
May minds have attempted this task. All have fail.
Your correspondent included.
(Small site, many moons ago, since surrendered its electrons back to the Great Disk in the Sky.)