My guess is that Santana will dominate the National League like Greg Maddux did in the mid-1990s and Randy Johnson did five years later. But maybe we shouldn't assume any limits.
Query: while the NL isn't nearly the hitters' league the AL is, how does the NL East stack up itself? Sure, Santana is going to pitch half of his games in a pitcher-friendly park, but there are going to be an awful lot of at-bats against Ryan Howard, Chase Utley, Mark Teixeira and Chipper Jones. Heck, even the lesser-lights on the Phillies look scarier because of that bandbox they call a ballpark. Though Florida and Washington aren't exactly scary, losing RFK is going to change the game in the NL East in favor of hitters as well.
This is not to say that Rob is wrong -- I agree that Santana is going to experience a considerable boost moving to the NL -- but circa 1995 Maddux and circa 2002 Big Unit approach the upper limits of what any pitcher is capable of, even one as talented as Santana.