For me its the UX. It just feels off, amaturish, messy. I can't really put my finger on it. I think the frankly crap fonts a lot of distro's choose to have as default dont help. And then the very "designed by a developer" feel to a lot of the UI.

And I know someones franticly typing away right now - yes, I am fully aware you can customise things, but out of the box it should be pretty damn well polished so that you don't need to.

Ubuntu's probably got the closest but it still just doesn't quite feel like they've nailed the experience.