The progression of string and racquet technology ever favors the baseliner over the volleyer. Players like Roddick and Nadal hit the ball so hard and with so much spin today that it is tough to beat them playing at the net.
Federer volleyed more five years ago than he does today because today players like Nadal are using lighter and more powerful racquets with poly-based string. The combination enables incredible spin and power.
Also, I don't rate Federer as a great volleyer. In this regard he is very much like Sampras. Both men are competent at the net, but neither was in the same league with McEnroe, Edberg or Becker.
While Becker was very close to Mac and Edberg in volley skills, he was much better than either of them from the baseline. Becker was much like Laver: lethal from the baseline and the net, and an excellent athlete. Everyone else has been either/or: good at the net, or good off the ground, but not truly gifted at both. Plus, Becker had a great serve and return game.
Sampras was pretty good off the ground (excellent forehand, but average backhand), was competent at the net, was a terrific athlete. What made him a great champion was his serve. Without the great serve, Sampras might never have won a single GS title. For example, he never came close to winning the French, the one major where his serve couldn't carry him to victory.
Federer is geat because of his fantastic footwork and balance and excellent athletic ability.