One thing that doesn't often get mentioned is that a Pi is a pretty decent way to play around with Risc Os. Probably better than digging up some old Archimedes/RiscPC or the overpriced replacement machines.
Risc OS is seriously underrated. It's so lightweight. Makes it super easy to play around with low level hardware stuff. It was made at a time when a couple of megabytes of RAM was huge and processor speeds were measured in low double digit MHz so it absolutely flies on the Pi.
It's not that it's underrated, it's mostly unknown.
I think the licence would make a lot of people skip over it; it seems to have far fewer features than Linux as well. I'm not saying it isn't cool, but reading their site and Wikipedia nothing is jumping out at me.