To point 1, Minecraft chews memory and processor for breakfast. I understand that some of the larger server have thousands of players logged in simultaneously over huge areas in a minecraft world, so I can imagine 4-5 digit hosting bills.

To point 2, you're hosting a server with lots of attention, one that is DDoS friendly, and you've got the eyes of lots of young technical people. In the world of one-click DDoS, I don't doubt it can pose an issue quickly.