If someone asked you what a great pitcher needs to do to win ballgames, you would surely respond with things like missing bats, keeping the ball down, avoiding mistakes and locating pitches. You know, things that can help to prevent the kind of hard contact that usually leads to extra-base hits. But walks are often at the core of any good offense's output, and the best pitchers in the game are able to stifle that in order to limit the damage that hard contact can do. Many pitchers can miss bats or keep the ball on the ground, but the very best pitchers are able to combine those kinds of skills with the ability to avoid handing guys free passes.
Just take a peek at the current ERA and FIP leaderboards. There's only one pitcher in the top-20 for ERA among qualified starters that has a walk rate above 3.1, and that's Oakland's Gio Gonzalez. Of those 20 pitchers, 7 are walking less than 2 guys per 9 innings. You see strikeout rates all over the place, ranging from Zach Britton's 5.02 K/9 to Tim Lincecum's 10.73 K/9. You see groundball pitchers (seven pitchers with GB rate of 50% or higher) and flyball pitchers (five pitchers with GB of 38% or lower). These guys have a variety of different styles. But the one common factor between the game's most successful pitchers so far this season has been the ability to avoid walking guys onto the bases.
And things are no different with FIP. Chad Billingsley has the best FIP for a pitcher that's walking more than three guys per nine innings, and he's currently got the 23rd-best FIP among pitchers with 40+ innings pitched. There isn't single pitcher with a walk rate that's higher than 3 and an FIP that's lower than 3.
What you really see here is how the ability to avoid walking guys is what really sets apart the good pitchers from the great ones. Lots of pitchers post strong strikeout numbers or avoid giving up fly balls, but being able to accomplish that while locating your pitches and keeping guys off the bases is what differentiates the elite from everyone else. If you're able to consistently miss bats even though you rarely miss out of the zone, or get guys to swing-and-miss at those out-of-the-zone pitches, that's a sign of a great pitcher.
So while people usually look towards the sheer dominance of velocity and strikeouts when evaluating the best pitchers in the world, that's not really the best way to identify the truly great pitchers. There are lots of decent pitchers that thrive on raw stuff and preventing hitters from getting the bat on the ball. Most of those pitchers also have a nasty habit to hand guys free passes as a result of their hard-to-hit ways. The great ones, though, find ways to get those guys to swing-and-miss without conceding numerous pitches.
Striking guys out is great, and it's usually necessary for any pitcher to succeed. But the difference between good pitchers and elite ones comes down to the walk; the good ones regularly give them up, the elite ones rarely do.