In probably the most intriguing pair of Rookie of the Year winners in memory, the two key Rookies for the World Series combatants both took home the award for being best in their class.
Buster Posey, a highly touted May callup, stunned the National League by posting a .305/.357/.505 line (.368 wOBA, 131 wRC+) in just over 100 games, belting 18 homers. Posey did cool a bit in postseason play, but with a .288/.354/.390 line, he was hardly a detriment to the Giants lineup. Posey's excellent play in 2010 created an internet catchphrase, "Posey'd". Even Keith Law was tweeting it.
On the other side of the Series, Neftali Feliz saved 40 games for the Texas Rangers with a 2.73 ERA (2.96/3.68 FIP/xFIP), striking out over a batter per inning and a 3.94 K/BB. Feliz was a major impact on the Rangers' season, coming into any leverage situation and dominating (he posted a 2.05 FIP in High Leverage situations - that's really really good), and accumulated 3.35 WPA on the season.
Feliz will likely be closing for Texas in 2011, per the Fort Worth Star Telegram, but should the Rangers decide to transition Feliz to the rotation, they could draw even more value from their young stud.