It's not really that the computer itself is retro, it's that people use it for retro computing because it's pretty much the fastest Mac you can get that can still run Mac OS 9 (1999). People then run games and apps from systems 1-9. So people buy this computer from 2005 to run an operating system from 1999 to run apps from 1984 to 1999.
Yeah for me OS9 is pretty retro - Classic Mac OS was great with some awesome apps many of which didn't make it into OSX and using the classic emulator in OSX doesn't help the experience.
The Mini is a great compact hardware platform to run them on. I have four minis that run various Mac OS generations.
