Take the Dell 5K screen for example. If you use Windows DPI scaling to make the OS actually usable, some elements won't be scaled up, explorer quick buttons for example. The whole Windows UI core is a relic from the 90's that has simply been patched up for years.
Been running Windows 10 preview on VMware for a few months. It's very promising, but some utterly retarded Windows 8/8.1 features are still in it, like not being able to use black window borders since the fonts won't dynamically invert their colour.