Consider a thought experiment...would Messi be as dominant in the MLS, or is a good portion of his success based on the quality around him at Barcelona? If you get perfect passes at a high rate, it is easier to score.
So many public faces have, not exactly flopped, but were less than expected. Kaka was fun to watch on Orlando because he never took the easy pass. He understood that when you work so hard to receive the ball in front of the goal outside the 16, you don't immediately pass it wide again like so many MLS players choose to do. And yet, Kaka was underwhelming in MLS. I would say David Beckham was underwhelming in MLS. It was great to see 80 yard cross-field passes dropped right on a guy's foot, but he was hardly carrying games for LA.
On the flip side, did anyone really know who Giovinco was before he came to MLS? What about Josef Martinez? And yet these players had two incredible seasons that left jaws dropping.
If you ask 100 Portland fans who their favorite player is, 80 of them will say Diego Chara, a midfield destroyer.
As for USMNT players, they are all a bit disappointing in the MLS. Howard, Bradley, Altidore, Bedoya are all just above average on their teams. I would say Beasley is the most popular USMNT player in MLS at the moment. What would really generate interest are young MLS players getting capped by the USMNT, but many posters on this board have stated MLS is not adequate preparation for international level. I think guys like Lletget and Zardes and Ebobisse getting caps will help build interest in the long run, much like Quakes fans were very proud of he who shall not be named.
MLS has found a sweet spot right now unearthing South and Central American young players that go on to be a big deal. Players like Miguel Almiron sold to Newcastle for $21 million. Each time a player gets signed from below the border, each fan base is wondering if this will be the next big thing that catapults their team to the top.
Most US soccer fans, actual fans, are quite knowledgeable. They don't need a household name to generate excitement. Casual fans, if they are willing to dip their toe in the water of MLS, can take their lead from the experienced fans.
I remember going to the 1994 World Cup elimination game between Germany and Belgium in Chicago. My ND roomate's dad worked for Verizon who had corporate seats, but nobody wanted them, so we took them (for free). I knew some German players, but the guy sitting next to me tells me to watch for the guy in the red boots on Belgium, Enzo Scifo, who played for Torino and Monaco around that the time. And so my soccer world got a bit larger that day. Great game 3-2. There were three goals in the first 15 minutes.
Expansion is an interesting problem for MLS. There are too many good pockets of fans left in the country to deny expansion, but a league structure, and a player pool, can only support so many teams. We have to import the players from somewhere, and they won't be UEFA/CONMEBOL international quality.