How Do You 'Vet' an Employer?

Not-to-desperate asks: "There is lots of info around on interviewing when hiring but what about the other way around? What do you look for in an employer? Are the any 'minimum requirements' that should be met? Obviously if you haven't got a job at all, getting hired is the main criteria, but what if you're jumping ship so to speak? I'm thinking of stuff like better salary, work conditions, type of projects, possibility of on the job training, and so on."