> Perhaps the versatility is the killer feature.That is a thirst of it. There is little it does massively better than other options, in fact for everything I can think of there are better options, but it is powerful to do most things well enough.
An other third is the cost. There are not many options with a similar price/utility ratio, particularly when you count support (see point three).
The thirst third is support: up-to-date Linux builds supporting the hardware (a common complaint with other devices is old and/or buggy drivers that are a faf to build), community size & momentum, commercial add-ons, ...
> As a media box, you'd want a SATA port
Not for a media display box, which is what my currently active pair are used for. Local media storage is on the network in a box hosting many drives and doing other jobs too, and other media is remote anyway. And if you are using something for storage you want multiple SATA ports (I can't be the only one paranoid enough to apply RAID1+ to anything intended to survive the month!).
The Pi3 (and 2 for that matter) does admirably as a Kodi box, though it struggles with x265 (720p is fine though it drops frames on some encodes, 1080p is sometimes surprisingly OK in the winter but causes the thermal throttle to kick in after a while when the ambient temperature is higher) so I'm quite interested in the fact that the 4 seems to support this in hardware - I'll be keeping an ear open for news that Kodi supports its hardware support for that codec (and if it handles the commonly used format options well not just the baseline).
> router / pfSense without a second gigabit ethernet
You can use an external device, but yeah that does wreck the nice small form factor somewhat, adds to the cost, and you have the hassle of finding a reliable well-supported one. Though the main complaints I've seen for using simple SoC systems like this as a router (and one of the reasons why I've not got around to trying it myself yet) is not that 100Mbps is a limitation for most home users (anecdote: have ~76mbit down, ~17Mbit up, I know few here with much better) but that they don't have the umpf to keep up with that level of traffic, especially in both directions, with any degree of extra processing (i.e. being a VPN endpoint for a chunk of that traffic).