So far throughout MLB, 380 different pitchers have thrown at least 40 innings. Among them, Shawn Kelley's K/uBB ratio of 5.6 ranks #8, between Dan Wheeler and Javier Vazquez.
Kelley hasn't gotten a whole lot of attention this year. For one thing, he missed two months with an injury. For another, he's been overshadowed by David Aardsma's success and Mark Lowe's personality. And lastly, he's just not that sexy to watch - he pairs a mid-90s fastball with a mid-80s slider and misses an average number of bats. He doesn't punch a ton of guys out or make them beat the ball into the ground, and if you just watched him once or twice without knowing who he is, you'd think he's just another generic middle reliever.
The thing about generic middle relievers, though, is that when they start throwing a ton of strikes, they're not so generic anymore. While the ability to throw a lot of strikes doesn't slap you in the face quite like strikeout stuff tends to do, if you can miss some bats and locate as well as Kelley does, it's really, really hard to be bad. Wheeler's a decent comp, actually - his stuff isn't great, but he makes up for it by not walking anyone. If you'd prefer, you can think of it as the Kevin Slowey* model in relief, but the take-home message is that avoiding free passes can compensate for an awful lot of drawbacks.
I don't think Kelley has a future as a shutdown reliever. At least, assuming Matt Capps doesn't count as a shutdown reliever. But in a bullpen crammed to the hilt with guys who struggle to find the zone, Kelley's stood out, and you can bet he's practically a shoo-in to be back out there in 2010. Middle relievers are middle relievers for a reason, but as middle relievers go, Kelley's about as good as you're going to get.
* or pretty much anyone else who's pitched for the Twins in the last five years