When detaching the module panel, the floating panel is always in front and obstructing the view onto the rest of slicer. This is very annoying.
When the panel is detached and I click on a different window (e.g. the browser), Slicer disappears instead of moving back. To the uninitiated, this might appear like a crash. I have to click on the slicer icon in the dock, to get slicer back.
There is no tooltip on the detach and close icons for the module panel.
When the panel is closed, an button should appear in the leftmost side of the toolbar that says something like: "Module Panel". Right now it is hidden in the View pulldown and you have to know its there. Not very friendly for new users.
http://www.na-mic.org/Bug/view.php?id=1323 is still needed.

This is a minor inconvenience, which would be probably hard to fix (hardcoded Qt behavior).

Agreed with the above comment. Introducing a custom solution would not follow normal QApplication behavior of QDockWidgets. Not following QApplication convention could be even more confusing.