Actually, not much. But there's plenty wrong with Gnome, KDE, Cinnamon, Xfce, LMDE and other third party additions. Linux itself is a good platform and it works very well on servers that don't have any keyboards or monitors attached. But in order for normal humans to use Linux and visually interact with the system, a third party desktop environment must be installed. Like Gnome or KDE. And that's where problems begin. Major problems.