Home Field Advantage in MLS
Over the 10+ years of MLS, the average PPG for every team in every game is 1.39. That breaks down to an average of 1.72 at home, and 1.06 on the road. That difference of 0.67 is a pretty big deal. A team who played only at home would gain 21 more points over a team that played only away games over a 32 game regular season. Or to put it into more easily understood terms, the difference is about the same as between the Revs and the Crew last year.
This year we're at 1.62 so far. Here's a table for the entirety of MLS:
The first number represents the PPG (shootouts = draws), and the second is the PPG difference between the home and away preformance for that year. It basically represents what home field was "worth" that season.
There were a large number of draws in 1999, the worst year for home field. But, the year with the most draws was 2004, which is at the other end of the spectrum.
Overall, it seems like the years with more parity are when home field advantage counts more. Which makes sense, I guess. Because if you're not seperated by your abilities on the field, then something else has to do it.
The numbers for a 12 team league are slightly lower than when there were only 10 teams: 1.70 vs 1.75.
For comparison, some of the top Euro leagues in 2005/6: