It's essentially the same (unless you use one of the DIY kits with pre-charged lines) as a full sized split system (traditional AC unit.) You (should) pressure test your lines and vacuum them out before releasing the charge. You also have to get a create a really good flare connection.

In addition, running the electrical is usually done by the HVAC company and setting up the mounting pad for the outdoor unit.

I completely agree that it isn't rocket science, and it's well within the realm of a skilled DIYer with about $250 in specialized tools. However, I can see why an HVAC company charges as much as they do. For an average install (not dead simple), it's probably 5-6 hours of labor, they have to pay for the company equipment, vehicles, health insurance, permits (my township requires one for a mini), taxes, etc.