As far as I can see when Metallica dumbed down musically everyone else did too... something happened between AJFA and just after the Black album that saw a shift away from virtuosity and musical structure toward very basic drop tuned groove riffs and big budget production. Pantera were one of the frist to exploit the groove riff - but in a good way with great lead passages and complex riffs thrown in for good measure. A lot of young players now are going for the cookie cutter approach to songwriting - get a guitar, learn how to play a power chord and all you need is a budget and a skilled engineer to prduce most of what's poular today. The really big production has killed the vibe too - the only BIG production metal album that sounds brilliant to my ear is the Black album - really amazing sound throughout. But I still prefer the aggression of the AJFA and the quirky sound of MOP.
I'm hoping they make big changes for the new album, hopfully the new bass player will inject some new energy and much needed inspiration. Also glad to see the back of Bob Rock - he had his run, hopefully Rick Rubin will get them back on track again. My wish list would be less 'melodic' singing from James and more aggression, more solos and complex instrumental breaks, more technical riffs, more speed etc.... basically they need to re-define the genre like they did with every album up to and including the Black album.