The simple part is UX people working with UX stuff, such as GTK or QT themes. The hard to impossible part is getting a typical open source UNIX desktop to look and behave as cohesive as Windows or Mac. It's a technology issue and not an asset issue.

As for beautiful UIs QT has made a step forward with the Qt Quick and Qt Designer which allow a more UX workflow for the UI as opposed to old school dialog/form design.