The fact that there really is no one dominant team in MLS works hugely in the league's favor. Year by year competition is seemingly wide open with virtually any team holding a shot to win. La Liga has Barcelona and Real Madrid, but is anyone else going to challenge for that title? Nope. Same can be said of the German Bundesliga. Bayern Munich are on top, and while Borussia Dortmund has been making noise of late, Bayern is the gold standard for success. In England you have the likes of the usual customers at the top, and a few squads occasionally chasing for that fourth Champions League qualifying spot.
We can point to the revolving door of players at many of the MLS clubs for the reason behind it, but for better or worse, right now this ensures open competition with each campaign. This makes it more exciting to watch for the casual fan.