Insert players...
Jack of all trades, master of none. Actually, the best insert players can rip it up from anywhere on a field. A good example of this would be Clive. He truly is a cut above.
Seriously, insert players. It's good to be a great front/mid/back player, but if you can learn them all, you can really adapt to anything thrown at you (i.e. you're mid and your back guy gets popped, you can fill or whatever).
-Mike