I think some of the players listed are probably getting too bad of a rap. Sure, some might not have ended up as a top 10 player in the world, but many were solid pros in stints (even if their potential might have marked them for greatness at times). I wouldn't put Rafael van der Vaart on this list, even if he didn't reach heights he might have been able to hit. He was pretty good for Spurs, and did show some things at other big stops on his career. Was he a top talent? No, but he did have a career that many footballers would happily trade for.
There are some other spectacular flame outs that never really did much of anything after even shorter stints. I think there should be some quantifying with some of it.