Most American sports leagues look at the regular season as a prelude to the postseason. Getting the number one seed is important but not celebrated, unless that team sets an all-time record doing it. Soccer, and the NHL to a lesser extent, does celebrate the regular season winner. In MLS that trophy is called the Supporters' Shield. The team also gets to go to the CONCACAF Champions League as a "Pot A" entry.
The all new Supporters' Shield is a massive, shiny plate with every winner engraved on it.
Same shot without narrow cropping forced by technology.
Another way that the Shield is unique and different is that its origins are not with the League. MLS supported and adopted the Shield as a meaningful trophy, but its first existence came from fans. It started on an email list in 1997.
The Supporters' Shield had its origins in the North American Soccer mailing list, as did so many components of American soccer supporters' culture in the mid-90s. For those who weren't a part of it, it was indeed special. A large, sprawling list of nothing but emails back and forth among a few thousand subscribers that maintained a higher level of discourse than any forum for the discussion of the game in this country than any medium of any sort that existed before or since. Really, I'm not exaggerating.
Much like the league the trophy's come a long way from the 90s.
The trophy (now the big platter version) is held not be the teams, but by the fans of the team that wins it. Locally Gorilla FC worked with the ISC on the new Supporters' Shield. Recently fans of the New York Red Bulls sent it out this way so that Gorilla could take the trophy down to LA. If Seattle Sounders win the Shield that night the team will hoist it above their heads in celebration and then the fans of the team will hold it until someone next year wins it.
At MLS Cup 2014 there will be a ceremonial passing of the Shield from the 2013 holders to the 2014 holders (in 2009 that ceremony was odd because it went from/to the same person). Representatives of the league, and maybe even the teams, will be present, but the Shield is for the fans.
Winning the Supporters' Shield comes with a lot of things - a CONCACAF Champions League spot, the number one seed throughout the Playoffs now including hosting MLS Cup, a place in the record books. It also comes with a bit more knowledge of fan influence in American soccer history.
Sounders FC can win this massive, shiny thing tonight. It takes just three points.