Its often necessary to split your screen between two different applications in order to reference one app while you're working in the other (e.g. working in Pages while you're referencing information on Wikipedia in Safari). OS X was never very good in this regard, and its an even bigger problem in Lion with fullscreen apps. Microsoft actually came up with a decent solution in Windows 7 by autosizing a window to fit half of the screen when you drag it to the edge. Well now that we have Mission Control why not allow full screen applications to be dragged and dropped on top of each other to create half-screen apps? It would sort of be like how you create folders in Launch Pad; you would enter Mission Control, drag one of your fullscreen apps on the other. Or drag a regular app onto a fullscreen app for the same effect.