[mr_dbr]

OS X looks good because it's visually consistent, something that cannot be said for Linux..

Just in terms of frameworks, on Linux there is (in common use today) GTK+, Tk, Qt, wx (and probably others). There's also different versions, and different themes (and the themes aren't interoperable, so if you have use a specific GTK theme, and start at a Tk application, they'll look completely different)

On OS X there is Cocoa, and that's about it. There is Carbon, but it's deprecated and used by very few applications

The default Ubuntu install looks good (colour-scheme preferences aside). It only looks bad when you start installing other applications - as soon as you install, say, a KDE application it looks horrible.. Not because that application necessarily looks bad, but because it's inconsistent..