Mac OS X prior to Lion is famous for limiting resizing of app window to just bottom right corner. This behaviour often frustrates users coming from other OS such as Windows. This changes with Lion. You can now resize window from any border and corner of a window in Mac OS X Lion.
You can also use the modifier keys while resizing window. Holding the Shift key while resizing constraints the window resizing to its current aspect ratio. Holding the Option key resizes the window from its center point.