Some of my thoughts on this;
I think at the top Federer would probably be strong enough overall to keep that spot. Thing is he isn't vulernable to many players on any surface. I don't actually think he would neccesarily win the majority of big matches but such a system rewards consistency against slightly less strong players. I also think Nadal would keep second place fairly comfortably. As has happened in tennis as it is he would dominate clay to a sufficient extent to build a commanding lead midseason and not be pegged back that far over the course of the year. I do think his win % on faster courts would suffer though as a result of being forced to play all the players.
Behind the top I think Djokovic would have an advantage over Murray. They seem pretty close in most places but Djokovic would be stronger over a long clay season and Murray wouldn't catch up during a grass season in which specialists are scarce. Whilst I see Murray as superior on the grass he wouldn't get that many more victories due to the weak field and Djokovic's overall competance. Del Potro actually might surpass both even if he could get enough wins in the short grass season not to leave himself floundering. In some ways he has shown himself to be superior to Murray and Djokovic at dispatching slightly lesser players.
Next is the chasing group. Would streaky players like Tsonga suffer? I think so, the weighting of points at the business end of tournaments rewards hot streaks greatly but a whole season doesn't. Roddick would extend a lead over the bunch imo, he is usually tough for a lower rank player to defeat. For me Davydenko and Soderling would head the current WTF group with Verdasco and Gonzalez behind and Tsonga after that.
Guys like Haas, Robredo, Monfils, Cilic, Stepanek, Ferrer, Berdych etc... are on the edge here. They would be fighting off relegation/pressing for promotion. I think a league system would favour the consistency of the hard working and defensively solid players but the long season would hurt them. That being the case I think late season pushes from strong indoor players like Berdych and Stepanek might do the trick with the grindy players being picked off with their weary legs a hindrance.
That's enough for now.