Interesting topic. I hope to see Dimitrov making some noise, really just because I enjoy his style of play. He seems to have all of the shots, so maybe if/when he's able to up the mental side of his game, he'll make a run. He's had some good results this year, though some bad ones too.
One thing to keep in mind is that (IMO) we've never seen guys who are as consistent as Fed/Rafa/Djokovic, in the history of tennis. By and large, they simply do not lose to underdogs. Compared to other greats that I've followed, like Sampras,Agassi, Becker, Edberg, Rafter, Courier - these 3 are a cut above in terms of consistency. So maybe it's unrealistic to expect anyone coming up to exhibit this level of consistency. That's my perception anyway, haven't tried to back it up with any stats.
There's no stats to back it up. You're right that the great players simply don't loose to lesser talent. They prepare like crazy. They put as much work in the non-match play activities as they do in their on court activities. They simply live for tennis. They live to be the best tennis player they can be. It's the most important thing for them.
But I don't see any way to quantify this aspect of a players character. They either get it or they don't. They may have all the talent in the world, see the two French top players, Gasquet, and Monfils, both as talented as the top players, but they like to enjoy life a little more than the top players. They are distracted from tennis. They have great results now and then, then loose early the next tournament or two or three. They are making millions, so who cares. Enjoy life.
Bill Belichick was once asked how he chooses players for the NE Patriots. His #1 thing was, they had to love football. Everything about football. This means putting football over nearly everything else in life. At best they live for getting better as a football player. They may not have the most physical talent, but they have great passion to make themselves better. There's a balance there. Finding a Michael Jordan (cut from his high school BB team, to become one of the greatest players ever), in amongst the hard working talent.
The same can be said of tennis players. The ones who love the sport over all else, and have the requisite talent, work harder than the ones who enjoy "life" over their sport. I think the great ones combine the talent, and the work ethic to maximize that talent. They get the most out of their abilities while they are young, then enjoy the fruits of that labor later in life.
Agassi is an interesting case in that he frittered away his talent for the first ten years of his career (life over career), then maximized every ounce of it for the second decade (career over life).