Which criteria do you usually use to decide what is more important to test and what is less important? I mostly have two parameters: what's the likelihood that the user will encounter the scenario under test, and what is the impact to the user if the scenario breaks. I will tend to test those scenarios with the highest occurrence rate and with the highest impact. Sometimes I would also add to the mix the probability that there is a bug in the scenario. So I would usually put more testing resources to areas that were not tested enough, have a more complex functionality or have a higher occurrence of bugs.
So which criteria do you apply to decide what to focus your testing efforts on?