In order (which is actually very hard to find):

  - Mission which truly helps people and focuses on the end-user and not simply the bottom line
  - Working with extremely smart/talented people, who don't take themselves too seriously
  - Robust development practices, to the point you can feel proud of your work and allowed to be the best in your field
  - Treats employees as adults and also truly value their work/life balance (i.e. happy employees are good employees)
  - Money

I'd take ~40k less a year (e.g. 85k vs 125k) if it meant being truly happy in my career with a company that meets the items on that list.