absolutely nothing, it's my preferred distro (and I have the grey beard to match). what I mean is that despite not using Ubuntu (or honestly even liking it that much), I give it a pass since it's the offspring of my preffered flavor.
Nothing, it's just a touch harder to use? I mean I use it on a old netbook and it works just fine. Did a net install and then loaded LightDM + MATE... Oh ... Yeah, there ya go, not quite as easy as Ubuntu. Still amazing for servers.
bingo, I enjoy the stabilty and simplicity for my servers but I wouldn't run vanilla debian for a desktop. ironically I tend to use Linux Mint for desktop which I guess is a grandchild then, since it's based on Ubuntu :). oops, I use LMDE, not Mint.. forgot that I switched a while back since it's directly debian based.