How many principles does it take to change a light bulb ... into a laser?

Quantum optics did not, and could not, flourish without the laser. The present paper is not about the principles of laser construction, still less a history of how the laser was invented. Rather, it addresses the question: what are the fundamental features that distinguish laser light from thermal light? The answers do, however, show, in a quantitative way --- involving, indeed, very large dimensionless quantities (up to $\sim 10^{51}$) --- that a laser must be constructed very differently from a light bulb. Some of this paper is based on material I use to introduce advanced undergraduate students to quantum optics. The theory presented is mostly quite simple, and yet it is not to be found in any text-books on quantum optics to my knowledge. The obvious answer, ``laser light is coherent'', is, I argue, so vague that it must be put aside at the start, albeit to revisit later. A specific version, ``laser light is in a coherent state'', is simply wrong in this context, since both laser light and thermal light can be described by a coherent state, though necessarily one that varies stochastically in space. Nevertheless, this perspective does reveal a profound difference between them, in that this description (a stochastically varying coherent state) is the only simple description of a laser beam. Interestingly, this implies the (perhaps new) prediction that narrowly filtered laser beams are indistinguishable from similarly filtered thermal beams. I hope that other educators find this material useful; it may contain surprises even for researchers who have been in the field longer than I have. But I cannot finish the abstract without answering the titular question: four --- high directionality, monochromaticity, high brightness, and stable intensity.