I feel that it has little to do with the level of competition he has faced that makes Cotto feared. It is the nature in which he has disposed of his opposition. I have yet to see a Cotto fight where an opponent leaves the ring with telling signs of punishment on his face.
Yes! You're right but in almost every fight he shows signs of being vulnerable (exhaustion, getting rocked here and there, gets cut, etc.), unlike Mayweather.
Cotto has a calm style
And that calm style is what makes Mayweather land fast speedy punches from all different angles and move/run away, shoulder roll, etc. Compare each other's speed, defense, footwork, who gets exhausted easier, who is more prone to getting cut, etc.