All UI control fonts are rendered using the wrong font with Qt 4.x on OS X Mavericks. Observe any Qt 4.x-based application such as VirtualBox, Parallels Desktop, Google Earth, etc.

This is because Apple changed the system font from "Lucida Grande" to ".Lucida Grande UI", which Qt Creator or Designer will actually show in the property editor as the default font name.

See developer.apple.com/wwdc/videos - "What's New in Cocoa", 23 minutes in, is relevant. You need to log in with an Apple Developer account (a free one is fine).

This issue may or may not affect Qt 5, because applications crash on startup. See QTBUG-31803.