JC has a stimulating post in which he writes that:

Judging player ability over time when player performance is a function of other participants in the game is not easy. For example, in sports such as running, where the outcome is measured by time, it is very easy to compare athletes over time using absolute measures of performance. The runner with the fastest time is clearly the best. However, sports such as baseball, where outcomes are a function of the relative performance of players, comparing abilities becomes much more difficult. While Babe Ruth was the greatest hitter of his era it does not mean that he is any better than the players in today's game. The pitchers of today differ from the pitchers of Ruth's era. While Ruth may have dominated in his own time, few would argue that this beer-swilling slugger would be the same player in today's game. But, it is possible that Ruth performed better against his competition than Barry Bonds does to his.

As they say in the blogosphere: Cliche the whole thing.