Power numbers are notoriously fickle. Plenty of players go through that one season where they flash a true home run swing only to fall back to the pack the next season. It's why people didn't give Jose Bautista full credit for his 54-homer season in 2010. Even though the general public may have missed on Bautista, the reasoning behind doubting a single-season power hitter without a previous reputation is sound. Home Run Rate takes 300 plate appearances before the sample even begins to gain reliability; slugging percentage and ISO can take full seasons.
There are a ton of factors that can go into whether or not a ball is a home run -- atmospheric effects (hello, Coors), wind, temperature, wall height, etc. ESPN's Home Run Tracker measures these effects and assigns a classification to each home run hit every season based on it's merit as a home run. These classifications include (from the site itself):
Just Enough: -- Means the ball cleared the fence by less than 10 vertical feet, OR that it landed less than one fence height past the fence. These are the ones that barely made it over the fence.
No Doubt: -- Means the ball cleared the fence by at least 20 vertical feet AND landed at least 50 feet past the fence. These are the really deep blasts.
Plenty: -- Everything in between.
"Just Enough" (or JE for short) home runs count as much as any others, but they're also an excellent predictor of which players have power that just won't last. In 2010, Casey McGehee hit 23 home runs, 15 of which were JE. He collapsed completely in 2011, hitting just 13 home runs in the same amount of games and slugging an embarrassing .346. Jayson Werth hit 13 JEs out of 27 homers in 2010 and fell to just 20 home runs and a .389 slugging percentage last season.
This isn't to say that all "Just Enough" home runs don't count -- Jose Bautista led the AL in JEs in 2010, for example, but at just 13 out of 54, he was actually below the league average of JE percentage. Overall, 1,521 of the league's 4,548 home runs qualified as JE, or 33.4%.
Here are some players who were well over the 33.4% league rate whose power may not be coming back for an encore in 2012:
Michael Young, Super Utility, Rangers -- 90.9%
It was a down power year for Young, who hit just 11 home runs after back-to-back 20-homer campaigns. One would expect a bounceback just given that information, but a look at the 11 home runs he did hit suggests otherwise. Not a single one of the 11 qualified as a No Doubt homer and only one as Plenty. At over 90%, Young is easily the leader in JE percentage of all players with double-digit homers and shouldn't be counted on for a rebound year in 2012.
Howie Kendrick, 2B, Angels -- 77.8%
Kendrick really came into his own for the Angels in 2011, posting a .285/.338/.464 line and looking like a key part of the new Angels' core next to guys like Jered Weaver, Dan Haren and now Albert Pujols and C.J. Wilson. Much of this was based on Kendrick's 18 home runs, a new career high, and similarly a career high .464 slugging percentage and .179 isolated power (SLG - AVG). However, 14 of Kendrick's 18 home runs were JEs, and as such his two-straight 10-homer seasons seem like better projections for the future than the new heights he ascended to last season.
Asdrubal Cabrera, SS, Indians -- 60.0%
Cabrera is the vogue up-the-middle power threat, socking 25 homers for the Indians last season. Combine the difficulty of hitting at Progressive Field with Cabrera's powerless past (18 homers in just over 1,600 plate appearances). However, Cabrera simply didn't hit the ball with the authority of a consistent power hitter, with 15 of the 25 home runs scraping over the fence as JEs and only two of the 25 getting the envied No Doubt classification. He probably won't slip all the way back to the single digits next year, but it's more likely than a return to the 20-homer club in 2012.
Martin Prado, UT, Braves -- 61.5%
Dustin Pedroia, 2B, Red Sox -- 52.4%
Miguel Cabrera, 3B, Tigers -- 53.3%
Adam Lind, 1B, Blue Jays -- 50.0%
Billy Butler, DH, Royals -- 47.4%
To see how the entire league did, scroll through this chart, ordered by JE rate. The size of the bar shows the number of JE home runs, and by mousing over the bars you can get more data.