setting core profile gets a blank background

MicPiano

I was following the cube tutorial the other day, and was trying to get it work with OpenGL 3.3 core profile. I have set my QSurfaceFormat in main correctly as you can see in the following code:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QSurfaceFormat format;
    format.setDepthBufferSize(24);
    format.setVersion(3,3);
    format.setRenderableType(QSurfaceFormat::OpenGL);
    format.setProfile(QSurfaceFormat::CoreProfile);
    QSurfaceFormat::setDefaultFormat(format);

    app.setApplicationName("textured cube");
    app.setApplicationVersion("0.1.0");
#ifndef QT_NO_OPENGL
    Dialog w;
    w.show();
#else
    QLabel note("OpenGL Support required");
    note.show();
#endif
    return app.exec();
}

I did also added #version 330 in both my vertex and fragment shader
here is my result
What did I do wrong?

another question:
My application is tend to be cross desktop platforms in the future, should I choose OpenGL 2.0 ES over OpenGL 3.3 Core?