The main strength a coach needs is the ability to communicate. A lot of players may have amazing skills with their feet but if they can't bring out those skills in others then they will fail.
Yes, a good understanding of the game is important but again, fail to get the vital information over to the players and it means nothing.
It's interesting that Mourinho, Wenger and even Fergie to an extent, didn't play a great deal or win trophies during playing days. Mourinho didn't even play.
You only have to listen to some players interviews to see they can't string a sentence together so they've got no chance at coaching.
Scholes might be ok, he seemed to be able to communicate on the pitch.