I always have a hard time dealing with the issue of sleepers. Some consider Rockies' OF Jeff Baker a sleeper - a player with some promise but no clear opportunity to fulfill it. Others consider OF Craig Monroe a sleeper - 20 HRs but the chance for more.
What sleeper-status depends on is the depth of your league. In 12 NL-only leagues, the Jeff Baker version applies.
In mixed leagues, the Craig Monore defintion applies.
For me, it is no fun trying to pick amongst the 15-20 HR hitters who will hit 28. It is important in those leagues that must do it, but I don't see much risk when the downside to a player is 15 HRs.
When the downside is playing the whole season in AAA, that is what gets my mind working.
There is another definition of sleeper and that is a player that is maligned by the sports media. Typically, these players do not have the particular skill that is in vogue amongst the sports media cognoscenti.
Someone like Juan Pierre fits this bill. His OBP is lacking, and he is always derided as replacable, but, when push comes to shove, he ends the year with 50+ SBs and no one complaining about his play - except the faux cognescenti for that lack of one skill.
Right now, the player heading towards that situation is Phillies OF Pat Burrell. In a Hot Stove Preview by Buster Olney in the current issue of ESPN, the always-worth-reading Olney says this:
This meme is as baseball mainstream as anything else, and, without actual games to dispel it, it will grow and grow in the next 4 months of the off-season.
God willing, he remains a Phillie, and I can scoop him up for $15-$20 at my NL-only draft!