Jake Arrieta started out the year doing pretty well, despite having a high ERA. He was throwing more strikes and missing more bats, which is always nice to see. Unfortunately, he wasn't able to stay healthy all year, as an elbow injury led to surgery and Arrieta missed the last two months of the season. It's possible that the bone spur in his throwing elbow had negative effects on his performance for parts of the season, but - assuming he starts 2012 healthy - there were some good signs.
Overall, Arrieta greatly improved his strike-out rate, from 4.7 K/9 to 7.0 K/9. It never made sense for him to have so few K's given his stuff, and more around league average seems like the "correct" range to expect going forward. Here are his whiff rates by pitch type:
Everything went up except for the change (which is the pitch he threw least often, and even less in '11 than in '10). By getting ahead in the count more often - he went from going to two strikes on a batter 43% of the time to over 48% of the time - Arrieta was able to get batters to expand the zone more. That can help allow the breaking-balls - which it looks like he may have become a little more consistent with - to be more effective.
Part of the improvements were due to platoon splits. Righties started to get more sliders, which they swung and missed more often (same thing to lefties, but they didn't see the pitch nearly as often), at the expense of fastballs. Lefties actually saw more heaters (with some closer to "cutters", having little horizontal movement), and their whiff rate on the pitch jumped from 8% to over 12% - matching the rate for righties (which stayed about the same - thus the overall increase from 10% to 12%).
The platoon splits on the curve were especailly interesting, as righties went from missing the pitch 28% of the time to 20% of the time, but for lefites the rate increased from 14% to over 33%. For sinkers the movement was in a similar direction, but to a much lesser degree. No wonder Arrieta went from having a very slight platoon split for strike-out rate to a somewhat large reverse split:
It's like Arrieta was two different pitchers; to righties he was Aaron Harang, and to lefties he was Fransisco Liriano. To left-handed batters he really worked away from them, which resulted in missing off the plate quite a bit (and hence the walks, as they declined to swing fairly often). If you combine the strike-out rate versus lefties with the walk rate versus righties, you've got a pretty solid pitcher (FIP ~ 4).
Beyond wakling 15% of the left-handed batters he faced, Arrieta also got taken deep with regularity. His HR/9 allowed was 1.9 against them, and a stilll bad (though better) 1.3 against righties. Some of that is surely flukey, given he improved his groundball rate from 42% to almost 46% (with much of the increase actually coming against lefties) - I wouldn't expect a 15% HR/FB rate again. Despite the ERA and FIP both going up (4.66/4.67 to 5.05/5.35), Arreita dropped his xFIP by over a half-run (5.17 to 4.52). There are some reasons for optimism going forward, even if he's unlikely to be much more than a #3 starter at the high end (that's like a #1 on this team!).