Sean Smith penned a neat article at Fangraphs yesterday, addressing the topic of historical defense-based positional adjustments.  He used TotalZone, his own fielding metric based on quasi-play-by-play data from Retrosheet, and compared how players fared at multiple positions within the same season.  Let's see the summary graph:

While a rolling average of year-by-year data would have been even better, there are some definite trends to pick out from the graph:

• Shortstop (the blue line at the top) has gradually seen less and less talented defenders.  One reason is that strikeouts are up, leaving fewer balls in play to turn into outs.  Another is that general managers have probably become less obsessed with a glove-only shortstop, especially with offense on the rise.
• Second base and third base (red and yellow lines) are pretty much equivalently talented positions, except for the 1980s.  Why the hiccup?  I'm not exactly sure.  From the 1960s and 1970s, defense at both positions is down the same amount -- evidently managers decided to make the change at third base ten years before second base.  That gap in the 1980s is probably why fans today have a hard time believing second and third base are equally important defensively.  That, and the fact that we're enjoying a Golden Age of offensive third basemen.
  
• Center field (green line) has slowly become a more important defensive position over the past fifty years, and the corner outfield spots (purple line) have pretty much held constant.  I'm not really sure why, although perhaps it just took a while for people to figure out that center fielders can make a much larger impact on the game than originally thought.