The Mets' outfielders are, for the most part, young. Andrew Brown and Jamie Hoffman are 28, Collin Cowgill is 27, and Jordany Valdespin and Kirk Nieuwenhuis are just 25 years old. The best parts of their careers are almost certainly ahead of them.
That doesn't mean that 2013 will necessarily be a breakout year for any of them, but over the last twenty years, 100 of the 1,793 players who had less than 400 plate appearances through their age 27 season went on to get at least 1,000 total appearances and continue contributing at the major league level. Many of them had long successful careers, while others had a good couple of years during their peak years before fading again on the other side of thirty.
A handful of those players are pitchers that had long enough National League careers to accumulate a lot of plate appearances, like Greg Maddux and Tom Glavine, but many are successful players and plenty are outfielders. It's easy to think of the Mets' outfield options as minor league filler, but it's not unheard of for players to achieve success at an older age than average or for players to take a year or two to adjust to the majors.
I call this the "Michael Morse List" because he had just 392 plate appearances through his age-27 season with an OPS of .764. He was a third-round draft pick acquired by the Nationals from the Mariners for Ryan Langerhans, who was one year older and had gotten much more playing time with less success. Morse then flourished with the Nationals, becoming a regular player and raising his career OPS to .839 and hitting 64 home runs for the Nationals over three seasons. That OPS is the highest on the Morse List.
There are plenty of other notable players on the list, though. Raul Ibanez only had 12 home runs before his age-28 season, and even that season wasn't very good. He now has 271 home runs with an .810 OPS. We know him now as a platoon guy, but during his best years, he could certainly hit lefties and was a valuable full-time player.
Matt Stairs is another slugger who had most of his success later in his career. His age-29 season in 1997 was his first with significant playing time for the Oakland Athletics, and he made it count. He'd hit 11 home runs in 299 plate appearances before that, but he finished his career with 265. Other players you may have heard of include Ryan Ludwick, Melvin Mora, John Vander Wal, Marcus Thames, Carlos Ruiz, and Paul Lo Duca. Many of these players have even gotten some MVP consideration.
This doesn't mean you can't still make jokes at the expense of the Mets' outfield, but remember that the outfield still has some potential and still doesn't have Jason Bay. Bay got 215 plate appearances last year and put up a .536 OPS. Every single player on the Morse List, with the exception of Glavine and Maddux, put up better numbers, and even Maddux had a 92-plate-appearance season in which he performed better at the plate.
The National League average OPS for outfielders in 2012 was .755. 23 of the 100 players on the More List have a higher career OPS. The Mets' outfield, the worst in the league except for the Astros — who have been banished to the junior circuit — managed a .705 OPS. 59 players on the Mike Morse List would've been better than average in the Mets outfield, with Andres Torres' career line being the 59th.
Torres is probably on the downward slope from his peak performance, his 2012 OPS falling .044 points below what his career numbers now indicate. Torres is an example of a player on this list who had a late start to his career, put up some decent numbers for a bit, and then quickly tailed off again. Perhaps this comparison applies to Marlon Byrd, but the other five candidates have the prime of their careers in front of them.
The Mets have a lot of unproven commodities in the outfield, but their potential is high. That possibility is extremely unlikely, but it's also premature to declare that the Mets' outfield as a lock to be awful. This isn't like Jeff Francoeur, who was already established as a bad outfielder before he came to the Mets, nor is it like Gary Matthews Jr., who was very clearly washed up when the Mets acquired him and probably shouldn't even have gotten the playing time he did. The Mets don't have Justin Upton or Michael Bourn, but with some growth, some luck, and some platoon time it's possible the Mets can get at least positive value out of their outfield corps.