When I first started really following soccer, back in college, that meant European soccer and, at the time, that mostly meant the UEFA Champions League. The Champions League is sort of a European super league, involving top teams from all over Europe (although it is no longer, as the name implies, comprised soley of champions). After a certain phase, the teams are drawn into the typical 16-team knockout bracket until we have a champion of Europe.
Unlike, say, the NFL playoffs, those knockout rounds are each two-legged: the teams play twice, once each at their home stadium, with the winner being determined by total goals scored. It is, in essence, one 180-minute game with a week-long (or more) halftime. So what if, after all that, things are tied up? In order to avoid going to the dreaded PK shootout, UEFA will award the win to the team that scored the most goals away from home.
That is the "away goals rule," as used in Europe and elsewhere. I'm not a big fan - I think it overestimates the difficulty of scoring on the road and, therefore, makes teams more cautious. That being said, I recognize that for a competition where the teams aren't seeded according to some regular season results it at least makes some sense. But where there is some ranking of teams heading into the knockout rounds, it really doesn't.
Since its inception, Major League Soccer has struggled to figure out a playoff structure that worked well, that captured the excitement of championships in other US sports while rewarding success in the regular season. It initially went with the baseball/basketball/hockey setup of a best-of-X (3, in this case) series, but that necessitates winners and losers in each game, which can lead to some issues in soccer.
More recently, the league switched to a two-legged series similar to what the Champions League does, with the higher seed hosting the second game and, if necessary, the extra time and PKs in case of a tie. Although it's produced some fantastic contests (such as the epic 2003 Earthquakes-Galaxy series or the 2012 DC United-New York Red Bulls series), it doesn't work all that well. It doesn't provide the higher seed with much of an advantage, making the regular season nigh-on meaningless. And, in the end, you can still have ties and PKs.
So now, with the new season just underway, MLS has announced that it will now tweak the system further by bringing in the away goals rule for the playoff series. While this will no doubt cut down on the dreaded PKs, it doesn't really solve the problem of the league's long regular season being next to meaningless. If anything, the away goals rule also means potentially fewer periods of extra time in the second leg, which is the only advantage a higher seed gets in the first place.
If MLS is serious about solving the playoff dilemma, it needs to look not to European football, but to good ole' fashioned American gridiron football. The NFL has the best postseason of any American professional sport. The playoffs last about a month, are super easy to follow (win or go home), and - as is particularly important here - rewards regular season achievement by allowing the higher seeds to host games. Scrape into the NFL playoffs and you're on the road the whole way. Cruise through the regular season and you get to stay home. Not that upsets can't happen and do, but they feel more earned if the lower seed goes on the road and gets it done.
It would be easy for MLS to import that system. Currently, 10 teams make it into the playoffs. That gives you four rounds - it even kicks off with a wild-card kind of round, giving the top seeds a week's rest at the end of the season. That round, by the way, is a one game winner takes all contest hosted by the higher seed. Why not just carry that format forward for the rest of the playoffs? No more midweek games that can't get on TV, no more cramped travel schedules. Easy! As a bonus, casual fans who might come across a game can get invested knowing that a result will occur that night - no need to wait for the second leg to see how things turn out.
I understand that part of the reason the league wanted multi-game series was to give each club an extra game to play, to fill the stadium and make some money. But lots of those games, with short turnaround and poor scheduling, aren't played in front of full stadiums anyway. Playoff games in front of empty seats don't look good for anybody. And, at any rate, if the league's going to grow up and away from such concerns eventually, it might as well go ahead and do it.
A one-game playoff structure isn't perfect, either. It increases the chances of games going to PKs, unless MLS did something truly original and forced the teams to play until there's actually a winner. My personal theory is that if teams knew there was no way a game could end in a draw they would be more interested in trying to win it, rather than simply trying not to lose it and slide through to PKs, but I admit I have no data to back that up. Still, even a PK-strewn playoff system would be better than the multi-legged confusion the league has now.
It's one thing for MLS to want to play like the rest of the world on the pitch. But when it comes to how to structure the league, it needs to do what works best for its market. The NFL has shown the value of a one-and-done playoff format. Why reinvent the wheel?