Qt designer provides a powerful interface for making Qt GUIs, including python GUIs (PyQt4 and pyside), much more powerful than matlab's GUI designer. However, it lacks one GUI element that Matlab has: a plot element. New GUI elements can be added to Qt Designer by means of plugins (pyqwt, which is unfortunately unmaintained, does this).

Since Matplotlib provides PyQt4 and pyside interfaces, it would be great if it also offered a Qt Designer plugin so matplotlib plots could be easily embedded in PyQt4 or pyside GUIs. I think python(x,y) has such a plugin that might be used as a basis for an official matplotlib one.