X window managers support a variety of placement algorithms for new windows: everything from asking the user interactively right up to smart placement algorithms that attempt to find an open space of the right size. However, they still don't always get it right. My pet hate is the behaviour triggered by the GIMP's crop tool: when you click on the image to drag the rectangle, it opens a dialog saying how big the area you've selected it. Many window managers try to place the dialog over the mouse pointer, which means you can't see the area you're trying to select.
The extended hints that most window managers understand these days already support hints to say where edge-of-screen docks are: new windows can thus be placed so they won't cover the dock. I'd like a new hint that allows an X program to specify an exclusion area anywhere on the screen, so that the GIMP can prevent new windows from opening over the image I'm working on when I select the crop tool.