NT 4 code was already leaked almost full back in 2004. You can still find it with relative ease if you know where to look, or search for certain keywords in the code.
UX, speed, simplicity, lightness. Applications ran without talking to the internet, asking if it's ok to run it, telling me it's probably unsafe to run it, telling me I need to update for security reason, telling me I should play candy crush, letting me search my files without adding recommended noise somewhat supposed to be relevant to what I did 3 days ago. I could go on. I just want to stare at a blue flat colour knowing tomorrow it will be just the same. /s
Depending on the "Distro" and compile options/build flags/included libraries MATE is as fast and light, since at least 6 years, actually. I think the first distribution which showed that to a general audience was LMDE a.k.a. Linux Mint - Debian Edition.