A couple weeks back, I wrote a piece on how the offensive environment had changed over the last decade or so. I posited that we should adjust our usual milestones according to the lower run scoring environment. It's pretty obvious we should not only shift our assumptions of offense, but we should also consider the effect on pitchers.
When I was in my formative baseball years in the early 2000's, I learned that an ERA in the mid-4's was pretty solid. Even knowing the change in run environment, I never really considered how it would affect pitcher's statistics. Well, the "average pitcher" has changed quite a bit since then:
Year ERA / Kper9 / BBper9
2012 3.97 / 7.5 / 3.2
2011 3.94 / 7.1 / 3.1
2010 4.08 / 7.1 / 3.3
2005 4.29 / 6.4 / 3.2
2000 4.77 / 6.5 / 3.8
Even though I do prefer RA vs. ERA (which doesn't account for subjective error scoring) I chose ERA just because it's more familiar for me and probably for the reader. What you can see over the past dozen years is average ERA has dropped dramatically by 0.80 points. It's interesting to note that an average ERA these days is under 4 (actually 4.03 in the AL, but the idea holds). Also, you can see there are big improvements in strikeout rate and walk rate. You might wonder how much the saber movement influenced this change, as a major tenet of their canon is to get away from "pitch-to-contact" pitchers and towards strikeout pitchers.
It's not right to attribute the huge change in the average ERA all to strikeouts and walks, as there are thousands of potential reasons for this. But one more aspect that's pretty easy to look at with numbers is the defense behind the pitchers. The importance of defense has been emphasized in the last five years or so, so maybe that is part of the reason for the lower run environment.
Year Defensive Efficiency
From this data, it looks like defense did improve from '99 to '02, but then plateaued from then on. Even though it's tempting, it's dangerous to attribute to changes in ERA just to defense from '99 to '02 and the change from '02 to '12 on just strikeouts and walks.
Regardless of the reason, strikeouts have gone up, while ERA and walks have gone down. ERA's in the low 4's aren't good anymore and mid-3's aren't as spectacular as they once were. Clearly, the benchmark for pitching has been raised. Go forth and do what you will with this information.