IMO, it's more than just tactics. For example, I play far too timidly in the openings, and conservatively overall. When I was younger, I was much more aggressive. Aggression spooks a lot of people, and they just give up, instead of trying to find the flaws in your play. Playing a lot helps build up good instincts. My brother has played more than 1300 games, for example, and was stuck around 1300 for a long time, but now is around 1575 or so. And then there is strategy. I'm crap at the opening, too, but if I can survive them, I do OK, but that's almost a tautology.
What makes a Class A - Expert Player?

But seriously, I can't answer that question... I really don't know. Maybe the experts seem to have a better "quality" of moves, and they blunder less often than A-players? That's at least my impression. But the levels of play are close enough such that natural fluctuation can mask playing strength differences... IMO
@DrCheckevertim gave the link today to this old forum thread. Time to reactivate it. Such an interesting reading.