Certainly there are many very talented drivers racing today, but the biggest difference between drivers such as Moss and Fangio was that unlike their contemporary counterparts, they would race just about anything with at least four wheels, and sometimes with only two. And they did it mainly for the challenge of competition, not a huge paycheck like they do today.
Today, if a driver from IRL or CART hops into an IROC car and races at Daytona, it's a big deal. Or when Tony Stewart moved over to NASCAR, that was also a big deal. Drivers like Dan Gurney, A.J. Foyt, Phil Hill, Graham Hill, Fangio and Moss drove many different forms of auto racing, from Saturday night Sprint cars, to F-1, to Nascar, to Sports Cars, etc. Can you imagine someone like Montoya or Shumacher running the Daytona 500, or the World of Outlaws??
As good as today's drivers are, you really can't compare the two generations together. Take away the huge multi-million dollar Cart or F-1 contracts and see just how motivated those drivers become if they had to hold down a regular job and have to race just for the thrill of competition. Many of the old drivers are making more money now signing autographs than they ever did racing.