  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 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 more specific,
quantum theoretic, version, "laser light is in a coherent state", is simply
wrong in this context: both laser light and thermal light can equally well be
described by coherent states, with amplitudes that vary stochastically in
space. Instead, my answer to the titular question is that four principles are
needed: high directionality, monochromaticity, high brightness, and stable
intensity. Combining the first three of these principles suffices to show, in a
quantitative way --- involving, indeed, very large dimensionless quantities (up
to $\sim10^{51}$) --- that a laser must be constructed very differently from a
light bulb. This quantitative analysis is quite simple, and is easily relatable
to "coherence", yet is not to be found in any text-books on quantum optics to
my knowledge. The fourth principle is the most subtle and, perhaps
surprisingly, is the only one related to coherent states in the quantum optics
sense: it implies that the description in terms of coherent states is the only
simple description of a laser beam. Interestingly, this leads to the (not, as
it turns out, entirely 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.
