MPL 2.0, copyleft, and license compatibility

In part one of my Mozilla Public License piece, I mentioned license compatibility as a major feature of MPL 2.0. In fact, it's such a major - and complicated - issue that it warrants its own explanation. First, a disclaimer[:]

Exactly what it means for two open source licenses to be compatible is a complicated topic; so complicated that the most sophisticated free and open source license wranglers often disagree on the nuances. This article can provide only an abbreviated overview, and will inevitably have some things that not everyone agrees with. Apologies in advance!