Assuming the player has a basic game to start with:
1. Second serve that enables the player to get an advantage in the point.
2. First serve that is a weapon -many serves not returned at all.
Having #1 enables #2. (Look what happens to Verdasco when he starts doubting his second serve)
3. Block serve return that can be placed anywhere on the court (see Federer).
4. Ability to take the ball on the rise effectively (enables court positioning)
5. Ability to hit effectively on the run (defense to offense).