They would have won a bunch of World Series titles, that's what.
The Tampa Bay Rays used the first pick of the 2008 draft on high-school shortstop Tim Beckham. Four picks later, the San Francisco Giants nabbed catcher Buster Posey from Florida State University. Posey has helped the Giants win two out of the last three World Series titles. He won the National League Rookie of the Year in 2010 and the NL MVP and NL batting title in 2012. He's hit .312/.381/.503 thus far in his career with the Giants. He leads all 2008 draftees in wins above replacement (WAR).
Beckham is repeating the Triple-A level this season. He's currently hitting .273/.335/.384. He's hit .265/.331/.380 during his five-year minor league career. He's yet to make it to the big leagues. He was also suspended 50 games last season for testing positive for a drug of abuse.
Tampa Bay is arguably the most well-run organization in Major League Baseball. Despite perennially having one of the game's lowest payrolls, Tampa Bay has been a contender in baseball's most competitive division for six straight seasons.
They are currently 32-26. Last season, they barely missed the postseason with a 90-72 record. In 2011, they eked their way into the playoffs on the season's final day with a shocking come-from-behind win over the New York Yankees as the Boston Red Sox completed their September collapse. Tampa Bay went 91-71 that year, but were ousted by the Texas Rangers in the ALDS. They went 96-66 and won the AL East in 2010. However, they were beat in the ALDS by the Rangers that season as well. The Rays went just 84-78 and missed the postseason in 2009, one year after going 97-65 and losing the World Series to the Philadelphia Phillies.
If Posey had come up for the Rays instead of the Giants in 2010, perhaps they would have won the World Series that season. Perhaps Posey never gets injured in a home-plate collision as he did with the Giants in 2011 and the Rays win it all again. They certainly would have made the playoffs last year if they had Posey's MVP production in the middle of the lineup. They would be favorites in the AL East right now. The Rays would probably be working on an extension to the cheap contract they would have signed Posey to after his rookie year. A one-two punch of Posey and Evan Longoria in the middle of the lineup would be among the game's most formidable. I would probably be the leader of the Tampa Bay Bandwagon right now.
Tampa Bay general manger Andrew Friedman hasn't made many mistakes during his career. Missing on Buster Posey in the 2008 draft was the organization's biggest blunder in his tenure to this point. With the draft being such a crapshoot, it's hard to call it a blunder with a straight face. Posey's high bonus demands and questions about his power ceiling and defense led the Rays to go with Beckham. However, I certainly wouldn't have selected Beckham over Posey.
I'd have taken Justin Smoak.