I look for guys with good ratios, high k/9's, but also closing opportunities (especially as handcuffs). I don't want to roster guys without potential opportunities because hot middle relievers emerge every year out of nowhere to help with ratios and k's. My list:...in no particular order.
I definitely agree with looking for guys with high k ratios....but I don't necessarily agree with the second part of your statement, not rostering guys without closing opportunities.Of course a lot of the decision to roster/non-roster middle relievers depends a lot on your league makeup, how deep your benches are, and how often you can make transactions.While it's certainly true that there are a few hot middle relievers that seemingly come out of nowhere each year - there are quite a few that year over year have helped my teams considerably. Off the top of my head, Scot Shields, Octavio Dotel and Scott Linebrink were awesome in that they usually got a handful of saves, a handful of wins, great number of strikeouts and good ratios. In daily transaction leagues, you could plug those guys in and make up k's, while helping your ratios and get a few wins/saves here and there to put you over the top. This year, I've got about 4-5 guys that I'm going after in just about every league. Guys that I pass on our generally journeymen who just came off a great year, or younger guys, moved to the bullpen who have never really closed before.Some of my buys

an Bard - Red Sox - this guy is not going to close, barring an unexpected injury to a closer who's been rock solid, but his stuff is great and he strikes out a ton of guys. He's been groomed for the bullpen, so this is not exactly a younger guy who got moved out there, and he has a year under his belt pitching in end game situations. I only expect him to get better.Brandon League - Sea - this is a fly under the radar guy that I really like a lot too. His k numbers have been consistently good, he's pitched in the bullpen for awhile, and he moves from the Blue Jays who are in the toughest division in baseball, to the Mariners, great pitchers park, and should be very competitive this year. Huge ++ in my opinion.Some guys I'll pass on:Michael Wuertz - Oak - this is the journeyman, one hit wonder that someone else will jump on early. He came out of absolutely nowhere last year to post phenomenal numbers. These were quite out of line with years past, and he's 31 now, so its not like he had some growing pains in his early 20's. I don't think he'll fall apart this year, but, I have to think he can only go down from last year.Matt Thornton - Chi - another guy who had an awesome year last year who is being bought to early. Unlike Wuertz, he's had decent year over year numbers, however, this is a guy that was predominately a lefty specialist, who had the advantage of only being brought in for situational outs. Often he was spared the late inning pressures that guys like Linebrink, Dotel and Jenks handled. Now he is being counted on heavily, late in games, and I think this may be an issue. Let someone else jump on him, although Thornton's upside is pretty high so if he falls real late, definitely worth a pick.And a sleeper pick:Tyler Clippard - Was - this guy falls under a) younger guys moved to the bullpen, and b) guy who out of nowhere put up great numbers. But, at only 25, and in a pitchers park, who knows who the real pitcher can be. His K/IP numbers are solid and he keeps it in the strike zone. I'm not sure if he's better suited as a starter or reliever to be honest, but he could be pretty good at either, imho.