One out of roughly every 9 hitters is different. Instead of pitching to a pitcher, you're pitching to an actual hitter. In some cases, a very good hitter. (It's not actually 1 of every 9, since sometimes the pitcher is pinch hit for in the NL, making the difference a little less that 1 in 9.)
If you look at history, it works out to something around 0.25 of ERA, although the gap has been trending down for a while now.
It's a factor, but not that much of one. And in any case, since we're talking about different run environments in the different leagues, how a pitcher compares to the other pitchers in his own league is way more important than how he compares to pitchers in the other league.
On average, every pitcher will give up more runs in the AL than if he were in the NL. It doesn't matter, since you're only trying to beat the teams in your own league for a playoff spot, and their pitchers will be pitching against mostly AL teams as well. What matters is how a pitcher compares to the pitchers in his own league.
Add a quarter of a run to a guys ERA if he's coming over from the NL as some sort of "close enough WAG", and then decide what that's worth.
Edited by USAFChief, 10 November 2012 - 12:43 AM.