During my studies at BrainStation, we were asked to conduct an analysis of an existing site's usability. Afterwards, we were asked to propose redesigns for the site based on the amount of positive impact it would make for users and the amount of time it would take for us to create. The site chosen, as you can probably guess, was craigslist.
In order to pinpoint issues that were on the site, each group member went through the craigslist site and determined its compatibility with Neilsen Norman Group (NNG)'s 10 usability heuristics.
These heuristics are a set of principles which serve as a set of best practices for user interface design.