Amongst 1000 other things Jason wanted to do a few side projects, which at the time the others were against (though they've all done considerable side work), and IIRC this is when it hit the fan.
Interestingly Jason was the biggest fan of metal in the group and wanted the band to do exactly what it did... put some thrash back into their studio output. The band has said where they are now is exactly the environment Jason wanted, but it took Jason leaving to get there.
Rob (their 4th bassist) is a great player, maybe better than Jason, but still I suspect we haven't heard the last of Jason & Metallica. He recently mentioned as recent as 6 months ago he was almost going to rejoin the group. Jason's jumped around a lot and now has taken Rob's spot with Ozzy on tour.
I get most of my recent info from
www.intersandman.com, the older stuff is from everywhere.