From where I sit, the gap between Murray and Djokovic is very small indeed. But for a poorly-timed-and-executed drop shot, or a bad forehand down the line, Murray might win that 1st set tiebreak and go up 2-0 in the match. Guys whose plus-skills are primarily offensive (Wawrinka, Federer, Tsonga, Del Potro, even Nishikori) fare very poorly against Djokovic. Murray is one of a handful who have the defensive skills, endurance and imagination to actually fight him to a draw for extended periods, and steal some matches.
I think the emotional evolution Murray needs is to channel his rage into greater focus, rather than self-doubt. That emotional skill is what I most admire about Serena Williams, frankly - when she knocks some dumb errors or plays poorly at important junctures, she may throw a hissy fit, but in so doing, she cranks up her own mental focus and goes into Beast Mode. All of a sudden her opponents are facing more aggressive shots that are better-struck, and it often turns the tide. The same raw abilities are totally inside of Murray - his serve is a big weapon, far better than, say, Nadal's; he's added great net play to his toolkit over the last few years, which was crucial to his Wimbledon win, etc. Nobody hits it more accurately when sprinting at full speed. And as you say, he's cut down the ranting tantrums quite a bit. But he still gets furious. I think that rather than trying to subdue that, he needs to just channel it better.
Federer and Nadal have Pedro-like calm in their big moments. You can just see how empty their minds are from point to point and shot to shot. Murray is more like Josh Beckett or Papelbon, needing to fire themselves up. Maybe he just has yet to find the right sports psychologist.