Mission and working environment have already been mentioned plenty of times, and are certainly important. One that doesn't show up a huge amount at the moment,

Mission and working environment have already been mentioned plenty of times, and are certainly important. One that doesn't show up a huge amount at the moment, but which holds huge weight for me (potentially trumping salary) is autonomy: to what extent am I going to have leverage to build useful things as an individual rather than a mostly-fungible "team member". I'm wondering whether some people have actually been a little scared off from advertising this because of the risk of being dinged for {bad,lack of} process.