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It's also worth taking a moment to consider how the population of pro baseball players may have changed over the years and how that might effect things as well...
Overall the US population is now much larger, and there are also substantial numbers of top notch players coming from other countries--Ichiro for example-- and of course you now have lots of excellent minority athletes who were previously excluded from competition. Of course there are now more teams than there were in 1956 and baseball has increasingly had to compete with football and basketball for top notch athletic talent, but overall, I'd say that MLB can probably recruit from a larger pool of talent now than was possible in 1956, and this likely allows baseball to be more selective-- a player who had just enough stuff to make it into the majors in 1956 might languish in AAA now, while your superstars of 1956 might be a step closer to run of the mill. (In keeping with the 'where are the .400 hitters' theory...)
It's also worth considering that free agency has allowed player's salaries to balloon, and the top players make a lot more money now than their predecessors did--apparently in 1929 Babe Ruth made only about $80k, which is still less than a million dollars adjusted for inflation. The prospect of higher payouts might convince more people to take a roll of the dice on a baseball career now than in times past, further increasing the potential pool of talent and allowing teams to be even more selective.
This, coupled with better conditioning, training--and steroids--is likely to make the average current player better than the average player fifty years ago.