From what I read and from what other manager told me it a balanced player is a player that have stats balance between the primary stat and the secondary stats like 100/70/70 to 100/30/30 and 100/50/50 behind probably the proportion the give the best result. On top of that you want to have your technique at the same number that aggression is to reduce penalty and be sure to train them in shooting or else they will be efficient in the opponent zone but wont be able get a decent shot on the net.

mortal kombat skill = very important

So a good player is a player that balance, with high attribute value (if you want them to help you right away), high quality of attribute and a high career longevity.