I used Baseball Prospectus quite a bit this year and, you're right, many of their projections are low, especially for young guys who have been great so far, like Victor Martinez and Miguel Cabrera.
What they do is use past players as comparibles and come up with expected numbers. I don't know of players as young as Martinez and Cabrera that have busted out like they have (well, except for Pujols).
At the half-way point of the season, I looked at the rosters of 5 teams in my league to see how PECOTA (BP's projection system) and I don't think PECOTA's too far off. They missed some players, like Victor Martinez, among others, but those guys have had amazing half-seasons, so far.
PECOTA projected much too low:
Scott Rolen, Melvin Mora, Victor Martinez, Matt Lawton, Miguel Cabrera, Matt Clement, Freddy Garcia, Roger Clemens, Ben Sheets, Oliver Perez.
~What projection system had those guys doing as well as they're doing?
PECOTA projected somewhat too low:
Miguel Tejada, Randy Johnson
~Randy Johnson was only projected to pitch 159.3 innings
PECOTA is very close to actual #'s:
Johnny Damon, Jason Varitek, Jeff Kent, Roy Halladay, Woody Williams, Javier Vazquez, Jason Isringhausen,
PECOTA projected somewhat too high:
Edgar Renteria, Jay Payton, Richard Hidalgo, Jeff Bagwell, Alfonso Soriano
~Hidalgo, for one, is coming around and could still reach his projections
PECOTA projected way too high:
Austin Kearns, Nick Johnson (both injured), Morgan Ensberg, Shawn Green, Marlon Byrd, Josh Phelps, Ryan Klesko, Arthur Rhodes, Mike Mussina, Joe Borowski (inj.), Matt Mantei (inj.), Pedro Martinez (off on ERA, close on K's and Wins), Kevin Millwood.
~A mix of injuries and players who didn't live up to anyone's expectations.
This is a sampling from 5 teams in my league, at the half-way point. One thing PECOTA was way off on was the Expos, who had some great hitting projections because of ballpark effects. Last year, the games in Puerto Rico were homerfests. This year, they aren't.
I'll dig into this more at the end of the year, with a full season's worth of stats.
Roger Angell: I was talking with Bob Gibson and I said: 'Are you always this competitive?' He said: 'Oh, I think so. I got a three-year old daughter, and I've played about 500 games of tic-tac-toe with her and she hasn't beat me yet.'