appearances and depth of WC runs. And Mexico has produced a number of world-class players over the years, such as Hugo Sanchez (one of Real Madrid's greatest strikers ever) and Rafa Marquez, who started for years at Barcelona. They have a bunch of players doing well in Europe now, although no one of Hugo Sanchez caliber.
Having said this, I agree that it seems like they should do better. They do seem to underperform relative to their population, as do many other countries (US, China, Russia, Canada, India, England, Spain before 2008, and so forth). I suspect it is mostly because of lack of investment and soccer infrastructure among the rural poor, relative to most successful soccer countries. Mexico is a large country with very rugged, mountainous terrain, and many of its poorest people live in small towns and villages far from any large city. It is expensive and difficult to implement sophisticated soccer development programs for those folks, so a large portion of the population, and especially the younger segment of the population, has little or no access to soccer development programs when they are young. By the time they move to bigger cities, many are just too old to develop into great soccer players.
During my lifetime I've seen Mexico progress and get consistently better (when I was a kid they were bad), so I believe it's just a matter of time until they break through and field a Golden Generation-type team.