E) Other: The job itself -- is this something I want to show off to my friends and family, or something I have a personal interest in, or is it just another job.
A) The people involved.
B) The amount you are paid. [devided by (Thanks for that Rene)] C) The hours you will have to put in.
D) The size and importance of the project.
Where I'm at, both in terms of location and step of my career ladder, it's easy to find people I want to work with but a lot of their projects are ones that I don't really want to support.
I'm coming to realize that, while my Sound Design CV isn't all that impressive, I don't need to rely on it to survive at this point in my life, since my day job isn't all that bad, and I've got enough material to build my demo reel. Now it's picking and choosing projects as I see fit, to work on things that I'll be happy with in the end as opposed to things that are just a paycheck.