One thing to bear in mind with Yahoo rankings is regardless of individual league setups, the rankings are all based on 5x5 scoring. So they will always overvalue players dominant in one category (steals, saves, homers, etc.) because those guys have more impact with just the five standard categories than in a league with more, like the Ks against for hitters mentioned above. So if you play in a league with more categories, definitely realize that the rankings might not apply directly to value in YOUR league. The rankings are also based solely on current stats and what the players has done so far, not what they are likely to do in the future, so you have to take care that you're not judging a player on stats he's already accumulated (and thus won't help your team) when he's unlikely to maintain that pace. The O-rank seems to try to balance the two a bit, but I've never found it to be too reliable.
"I'm telling ya, he jumped us. Gloves off, stick down, no warning. He challenged the Chiefs. Called us names. But Dave was there."
"Dave's a killer."
"Yeah, Dave's a killer."
"Dave's a mess."