Here are the major ones. A player needs to continually throw away his cherished skills and replace them with something better. This can cause havoc in staying sharp and confident. Most player would not dare to let go anything that has raised him to the current level and is responsible to keep him there. Even for the rare few who dare, the inner conflicts can become too much for the mind to bear.
Speed is the king, and mental toughness owns the match point.
It's an eternal struggle for a player to force himself to play fast and play gutsy. For ordinary players, physical exhaustion and psychological boredom are the major hurdles. You need to have a fast and sudden serve in your repertoir. And in returning a serve, you must be able to attack any kind of serve. But most players would avoid the whole issue and indulging themselves in rally practices.