Lot of interesting things in this thread from all parties. I don't really have much to add that hasn't been said before. For the most part, actually all of it, I have to agree with Bab and Luggy - the lack of variety really kills interest.
The one thing I guess Herc keeps bringing up that I see as the underlying theme is that if winning is the number 1 thing of importance, and you can factor out some main aspects of things that is a recipe for success today, why not emulate that? If you had to extract the top 3 successful attributes of the top 10 players, you'd most likely have some combination of: Killer forehand, Consistency through points and ability to maintain the consistency set after set, hour after hour which is apparently the new definition of athleticism, and probably the serve. Nowhere, and regardless of how often guys like Fed, Tsonga, Gasquet, Haas can work in their slices or volleys into their game, it only eases/helps their game, it's never the defining factors for their success. You take away the slice backhand and volley from Fed's arsenal and arguably he'd still be one of the best and ranked just as high as he is. So a young player like Cilic - who by the way I had a lot of hope for and still do like - who might have had the serve and the wingspan and the flexibility to do something more with his game, would rather learn to improve his fitness and up the consistency of his groundies than anything else. The most successful player and reliable player on tour apart from the top 4 these days is Ferrer, who's a prime example of someone who keeps it simple and wins a lot with it. Can you argue with that? It's tough.
That's just the problem restated the 100th different way. I don't have a solution. The day that variety actually becomes an important factor in someone's success, we'll see more people trying to emulate that. I think guys like Fed, Tsonga, Gasquet, Haas, Nishikori have a little bit of creativity that makes them more exciting to watch but apart from Fed, none of them have had the level of success that the player who wants to be the next big thing can look up to. And even Fed, the basis for a lot of his success, in addition to having an excellent serve, forehand and footwork, is that he is an excellent defender and supremely fit enough to hang around on court long enough in most situations to utilize his skills.
And that leads to my final point which has to do with another excellent point Bab brought up: changes in technology has actually helped clay the most. Of all places, clay is the only surface that has become more interesting over time as we have moved away from moonballing and there's somewhat more time for decent point construction and executing some variety if the players choose to. On the other hand slowing down the rest of the courts has put the emphasis on stamina and the top 4 and Ferrer have benefited by it by that single virtue alone. Each of the top 4 can outlast any variety, any power, and anything that the rest of the field can throw at them 99% of times because they are that much fitter and can hang in there.
Again, until we see some youngsters match that kinda stamina AND bring in some variety on top of that, OR have enough variety that it's enough to elevate them over their physically fitter superiors, I don't see what's there to motivate youngsters to approach the game differently. The alternative is that we have maxed out the limits of physical fitness and last-man-standing capabilities with Nadal and Nole, and those who follow have to find something different to mark themselves.