One thing OS X has struggled with is window management. Prior to 10.7 or so, most apps used the “+” button to show full window content; now they tend to expand arbitrarily or worse, take up the full screen (to appease Windows converts). I currently have ShiftIt installed, which emulates the Win+[direction key] for splitting two windows horizontally in the full vertical space. One thing I’d love to see is a way to “lock” two or more windows vertically and have a slider adjust horizontal space each window (think Windows 8 Metro full screen snap with fully flexible window size). That’s probably more of a fantasy than anything, but it’d be nice to see.