In American sports, four leagues dominate the professional level in terms of fans, revenue, and interest: the NFL, NBA, MLB, and NHL. However, come summertime, only one league dominates: MLB. Sure, the NBA and NHL Finals go into early June and the NBA draft is in late June, but overall, MLB dominates.
Now I'm a big MLB fan and think it is a great league. However, I know others who would disagree and say that MLB and their 162 game schedule makes it hard to follow a team and games have little importance (unlike NCAA Football and the NFL). Even the NBA doesn't come close to this number of games in a season (82 compared to 162). A single MLB game doesn't even compare to the importance of an NFL game, and understandably so. But in the summer, the only thing that's on is MLB. But there is an upstart professional sports league that plays in the summer and has around a 30 game season (equivalent to a NCAA Basketball season).
The MLS (Major League Soccer) plays throughout June and July and have only 30 games in a season. That makes each MLS game about five times more important than an MLB game in terms of meaning in the rankings. Now soccer isn't very popular in the US—at least not yet—but in other countries, like England and Spain, it's by far the biggest sport.
The English Premier League, the world's top soccer league, is followed worldwide by over half a billion people in over 200 countries. That's by far the most popular sports league in the entire world. Check out this video. The EPL is to England what the NFL is to the US. The US has a larger viewing audience potential than England and US clubs would eventually be able to pay as much or more than the leagues in Europe.
Some people might say soccer is boring, but they obviously have not been to an international match where the players are a lot better and the fans go crazy. The US has the money, audience, and potential for the MLS to become as big and important as the Big Four are now. And with 16 teams currently and future teams in Vancouver, Montreal, and Portland, the league is growing at a fast rate. The teams in Seattle and Toronto currently have a higher attendance per game than their MLB teams. And if the US somehow miraculously wins the World Cup, the MLS could be the next big thing really soon.
Watch some of the World Cup this year before you say soccer is boring. Soccer has as much action as baseball, and the MLB is big in the US. Remember this when the MLS is big in a decade.