Nice summary, HopelessN00b! I’d like to add the following, regarding the operation of two 2012 Essentials in parallel (which I wish I had known before I purchased two of them):
As you already mentioned, Essentials needs to be a Domain Controller, i.e. it must have the DC role installed and properly configured, otherwise it shuts itself down every 7 days. (If it’s installed but not configured properly it shuts down every 31 days.)
There can be only one Essentials on your (discoverable) network. If you run another one on the same network they’ll find each other (via NetBIOS?) and one of them shuts down.
You can, however, run an Essentials side by side with an Windows Server Standard (2008/2012 doesn’t matter) without problems and even connect their Domain Controllers, as long as the Essentials has the primary DC role.
BTW: In my tests, it turned out you actually can connect two Essentials’ DC roles to one Standard DC without them shutting down each other, but as only one of them can have the primary DC role, the other one will shutdown itself within 31 days (as it’s not properly configured regarding the license requirements).
Of course, if you’d dig yourself through the whole Essentials licensing documentation, you’d find those information, but if you’re like me think »I don’t need all those Standard features, I just want a small server for XY« this could easily get you in the same trouble as I got. Hope this saves someone some days (and money, as there is no upgrade pricing from Essentials to Standard.)