I just upgraded my Qt from 5.2 to 5.3. In Qt Creator I have now no application output at all: qDebug() and cout are not working. In "Build & Run"/"Qt Versions" it was set to 5.2 and I found that this problem may be related to bad library in use... so I've changed it to 5.3 (which I now have) but still nothing: program builds and works but doesn't let me use qDebug(). I'm using Arch Linux.
It turned out to be Arch Linux bug so really not a problem that could be answered here.