For all the belly aching, if you look at the serve and return numbers today, they match very closely to the numbers from 20 years ago. The best returners in the game are winning 30% of return games on grass, 35% on hard courts and 40% on clay. Same as 20 years ago. Points have probably gotten a bit longer, but again things change. Deal with it and move on people.
I think there is less variety because the ATP schedule highly discourages specialists from focusing solely on their favorites surfaces. If you want to maintain a ranking or score the big bucks, you need to be great on all surfaces. Gone are the days of Sampras getting six years at number one withing doing anything on clay. Gone are the days of Muster focusing almost entirely on clay, even skipping Wimbledon most years and going his entire career without winning a single match at Wimbledon.
There's also a certain Spanish Bull from Mallorca who serves as a strong deterrent against clay specialization.
So the result is that we have fewer underdogs pulling upsets on their favorite surfaces, and more consistency. It's harder than ever for a dark horse to break through and get that one unlikely Slam. There's no more room for Goran Ivanisevic and Muster and Chang and Noah and Gimeno. It means that winning all four Slams isn't quite as impressive as it would have been for Sampras. But it also means that breaking through at all is more special, because it is so rare these days. Del Potro is a perfect example--he only has the one Slam, but there have only been five total winners in his entire ATP career. So he really only needs the 2009 US Open to really stick out among his peers, whereas previous generations would have needed maybe two Slams.
Everyone will remember Del Potro for the way he won the USO. In fact, he may go down as the first man ever to foil a couple of 15+ Slam Winners. The more Nadal wins, the better DelPo looks.
Even Nalbandian and Davydenko's achievements are impressive, albeit often overlooked.