Both the Yankees and the Mets completed first halves that reaffirmed some age-old baseball lessons, while teaching a few new ones. The takeaway for each team includes:

METS: In a weak division, you can look good just by making an effort. Not all of Omar Minaya’s off-season moves were well-considered — the Mets could use Kris Benson back — but all of his competitors’ moves were worse or simply nonexistent. The Mets’ 18–8 record against the Phillies, Marlins, and Nationals was a big part of their early stranglehold on the division title.

YANKEES: Depth matters. The Yankees went through the off-season without addressing their severe weakness in outfield depth; instead, they resigned Bernie Williams and pretended Bubba Crosby could play more than a tertiary role. Meanwhile, inexpensive outfield reserves were available on the free-agent market.

No doubt the Yankees were lulled into a false sense of security by the extraordinary durability demonstrated by Hideki Matsui, Johnny Damon, and Gary Sheffield in recent seasons.The early injuries to Matsui and Sheffield and Damon’s season-long struggle to avoid the DL underscore just how dangerous betting on a trend to continue can be.

METS: Depth matters. When the season began, it seemed as if Jose Valentin had no role on the Mets. After a lost season with the 2005 Dodgers (.170 AVG/.326 OBA/.265 SLG), the 36-year-old seemed unlikely to recover enough consistency to make his power an asset. He was also behind David Wright and Jose Reyes at the two positions he had played in the majors (his experience at second had been limited to 19 games in 14 years). Besides, with Kaz Matsui, Anderson Hernandez, and Chris Woodward available at the keystone, he was even further down the depth chart.

Minaya and Willie Randolph thought it wise to take on Valentin as an insurance policy on their insurance policies, and the move paid off; finally getting a chance to play in May,Valentin has batted .294/.339/.575 from May 1 to present and has turned a lost position into a Mets positive.

YANKEES: Not all catchers age at the same rate. After Jorge Posada’s inconsistent performance last season (.262/.352/.430), many observers looked at his 33 years of age and decided he was on the downside of his career. This season’s .279/.389/.466 performance, right in line with his career numbers, suggests they were premature.

A late conversion to catching and his long fight to pass Joe Girardi on the depth chart has helped keep Posada’s mileage down, even with the heavy workload he’s had with the Yankees at times. Even as Posada ages, his walks and power will make him a viable player long past the time his batting average ceases to be competitive.

METS: Carlos Beltran is a heck of a player. When Beltran struggled with injuries and apparent insecurity last year, many who had prophesied that the outfielder was a small-town ballplayer who would be humbled by New York City and his inflated contract clucked knowingly. They should have waited to see if the injuries were the greater part of the equation. Beltran is having the best season of his career and is a huge part of the Mets’ success, giving them an impact bat at a position where few teams have one.

YANKEES: When there is no good reason that a pitcher should be doing well but is anyway, it’s probably not going to last. You could argue that the Yankees lacked alternatives, but the fact is that they would have been wiser to thank Aaron Small and Shawn Chacon for a job well done in 2005 and let them move on.

METS: It’s never Lima time, but it may not matter. Assuming health, which is more doubtful than ever at this point, the Mets have only two starters on whom they can count: Pedro Martinez and Tom Glavine. Steve Trachsel has been taking his turn in the rotation but has been unspectacular, and Orlando Hernandez is like Forrest Gump’s box of chocolates — you never know what you’re going to get.

On any given day, the fifth starter might be anyone. It might be you. However, as Baseball Prospectus has shown, the greatest indicator for success in the postseason is having three strong starters and a strong closer. That means that if the Mets can avoid a 1964 Phillieslike collapse, their pitching problems may cease to matter.

YANKEES: New Yorkers can still call themselves the country’s smartest baseball fans, but they do miss some things. Alex Rodriguez is an all-time great player having a very good season for the Yankees. One of the few areas in which the National League beats the American out on depth of talent is third base, where several players (including David Wright) are having terrific years. In the American League, no one is close to A-Rod.

Yankees fans would still prefer to boo him and, in a rare instance of deference to Boston, argue that David Ortiz is a better player without seeming to realize that until Big Papi shows he can play third base, the argument is moot.

METS AND YANKEES: Changes are coming. With no strong competitors in the league, the Mets have the luxury of gearing up for the next step, of being active in the trade market with an eye toward going deep into October. The Yankees, a very good team in a stronger league, are going to have to get better if they’re to avoid being shut out of the postseason. It won’t take much to narrow the gap between themselves and the Red Sox, but GM Brian Cashman will have to be creative to make it happen.

Mr. Goldman writes the Pinstriped Bible for www.yesnetwork.com and is the author of “Forging Genius,” a biography of Casey Stengel.