There is no one thing that makes a drive train good. Everything works together. You can have all low end components and, if everything is right, it'll work smoothly and silently. All Dura Ace but with a just tiny kink in the shift cable, not so good.

The trick is finding the tiny defects - a kink in the shift cable, a derailleur hanger that's not quite aligned, a cable housing that's not quite square. When the shifting is absolute crapola it's probably going to be something fairly obvious and it's easy to fix. When it "works OK, just not to your satisfaction," that's actually a lot harder to diagnose. Frankly, I think that most bike shops are more geared toward making the bike useable than to achieving super smooth drivetrains.