I had some more time to think about the plus/minus scores published at the Climbing the Ladder blog, and I posted a comment to the post a few minutes ago.  My critiques of the current version are that it uses solely 11v11 match situations, treats the periods of 11v11 play as one contiguous block, and doesn't consider the impact of the teammates or the opponents.   I do think it's a step in the right direction, and could be refined into a very useful statistic. 

I'm not going to tell you everything I said — I'm sure the other blog could use some hits (it deserves some with its extensive set of stats on MLS), so check it out here.