So the four top men's seeds reach the semis. This tournament has, in a way, typified the way mens tennis is right now. Leaving Ferrer aside, the only way anyone outside the top three can come close to beating one of them in a GS is by hitting a real purple patch (as Wawrinka did against Djokovic) and even then it doesn't guarantee victory. Other than that, the top three swat the others aside like so many bugs that seem to invade the courts in Melbourne.
Ferrer is in the semis because he has taken over the mantle Murray had until his 2012 breakthrough year - excellent player, knocking on the door but never quite regarded as in the same league. Unfortunately, Ferrer neither has the in depth game, confidence nor time on his side to break through into the real elite club which the top three have made for themselves.
I've deliberately left Nadal out of the above analysis as I'm talking about the here and now. If/when he comes back and if/when he comes back strong then the dynamics may shift.