League average for OPS tends to be around .750 or .760, with the leaders sitting on the high side of 1.000. At the other end of the spectrum, a few players will languish under .650, but .600 appears to be the real line in the sand. It isn't as though players don't fall below that mark -- even superstars like Joe Mauer (.582 in September/October of 2005), Josh Hamilton (.513 in July of '09), and Mark Teixeira (.559 in April of last season) have had a month or more when they struggle to reach .600 -- but dwelling there for a full year takes consistently poor play and the faith of a manager, even in the face of mounting ineffectiveness.
Since '00, just nine players have qualified for the batting title with an OPS of .600 or lower: Brent Abernathy, Brad Ausmus, Clint Barmes, Angel Berroa, Michael Bourn, Cesar Izturis, Nefi Perez, Nick Punto and Ramon Santiago.
Many of these players had one skill that kept them in the lineup: Ausmus was a veteran catcher who managed his pitching staff well, Bourn's speed is still among the best in baseball, and Perez and Izturis had reputations as good defenders, even if the metrics don't bear that out. Whatever their respective secondary skills, an OPS of .600 or worse means that you're likely to find their brush with baseball's basement counted among the worst overall seasons of the last 20 years. In Berroa's case, his '06 season ranks as one of the 20 worst seasons of all time.
An added benefit of this move away from a batting average-driven marker is that it frees Mendoza of his shame. It was always a bit unfair to name the line after him considering how miscast he was as a regular infielder in 1979 and the fact that he would have fallen 100 plate appearances short of the current standard for qualification with just 401, but someone new must bear his mark.
Step forward, Nick Punto, you have been chosen for such a moment as this: An OPS of .600 shall now be known as the "Punto Slash." His '07 "slash stats" -- the common statistical line of batting average/on-base percentage/slugging percentage -- was .201/.291/.271. Failing to get either his OBP or slugging percentage over .300 means that he wasn't getting on base at an acceptable rate and he wasn't advancing past first base if he manage to find a hole.
Punto illustrates the point perfectly: He's never posted an OPS at or above league average, but was typically able to walk a fair amount and save enough runs on defense to hold down a spot on a competing roster. In '07, the bottom fell out of Punto's offensive game to the extent that he fell 40 points shy of the line that now bears his name, but the perceived lack of replacements available to the Twins meant that he played a career-high 150 games even as he fell to the bottom of the OPS leaderboard. Just seven players finished with an OPS within 100 points of Punto's .562. Not to put too fine a point on it, but 14 players finished with a higher slugging percentage than Punto's total OPS.
To his credit, Punto did not let his struggles at the plate affect his defense.
"Nick flat out kept runs off the board. He was on SportsCenter every night," said Twins batting coach Joe Vavra. "He was a leader off the field and he still did some things with the bat. Sometimes teams also keep guys in the lineup because of depth and injuries. Keeping runs off the board is just as important as scoring them, and we had other guys around him that balanced out the lineup."
Indeed, whether you prefer defensive metrics or your own eyes, Punto was well above average at third base, but it's hard to comprehend the level of defense it would have taken for him to equal out his offensive issues that season. According to Baseball Reference,
With the exception of a few elite players -- the Pujolses, Crawfords and Beltres of the world -- there is an offense/defense tradeoff, that's part of the challenge of roster construction that every GM faces. There will always be players who derive much of their value from what they do with their glove, and that's fine, but it does put them at a higher risk for abject failure if the offensive prowess they have leaves them for an extended period of time.
It is possible for a player to hit at the Mendoza Line and still give his team enough value -- through defense and a high slugging percentage -- to justify a roster spot, which defeats the original intent of the concept. Once he passes the Punto Slash, however, it becomes virtually impossible for him to add enough value through the rest of his game to keep a team from looking for replacements.