1. Is the person I'll directly report to a jerk or the like (unfortunate when this person changes soon after you start...).
2. Private office with a door that I can close (if my productivity isn't important to them ... that's a bad sign. If they don't even realize the significance of this, e.g. of flow, even worse).
2. b: The office also has to be warm enough; I've had to quit two jobs because there was very cold air pouring down on me and that triggered an inevitable sinus infection. They didn't listen before the latter happened and didn't care much or at all after. In general, make sure the physical environment is tolerable for you and that they care about this sort of thing.
3. EMACS is non-negotiable, although that's never been a problem.
After that, the usual stuff, much of which is already well covered in other responses.
* Can I work for or with a person that can teach me something new? (Hey Marc, thanks!)
* Possibility to travel?
* Is my team cool?
* Is there a challenge? Am I forced to learn something new?
* Is the project/business at least somehow sane?
* Am I free about the where and when, as long as it gets done?
* Will I be able to get some shares?
more on a technical level:
* Do they follow best practices like version control or automated testing/builds?
* Am I free to choose my tools?
* How high is the wtf-rate when reading existing code? Or better: Do I want to spend the next x weeks digging into this? (Hint for rails coders: Do a 'rake stats'.)
Hope you will find your dream job soon. All the best.