Negative affectivity is a particularly important variable affecting the emergence and operation of burnout, and can be assessed with the PANAS. I would also look at relatively stable individual and situational differences, such as financial and job security, relational demands at work and home, and social support. If you want something a bit more unique, look at Kobasa's concept of dispositional hardiness, which psychology seems to have forgotten. It's a super cool individual difference. You could also look at status and power of the individual, which may affect how they experience and recover from burnout.