The parallels between the Social Gospel and the Emerging Church conversation are striking and interesting by the way - since you mention this also.  The biggest difference seems to me to be the way that these two movements conceive of the "state."  Social Gospel folks like Walter Rauschenbush were pretty optimistic in their pre-WWI worldview about actually "Christianizing" the social order.  They thought the nation state could be co-opted fairly well in some respects as a tool for accomplishing things that aligned with the values of the Kingdom of God.  And with this they had an overly-realized eschatology that it in my view is indeed taken too far sometimes by emergent folks as well.  

I'd like to think that the emergent church, however - if we can speak of such a thing - with its "missional church" friendliness, is much more suspicious of party-politics.  This would not be to say that Christians should remove themselves from participation in activism or the electoral process - not at all - but there's a greater appreciation in the post-modern Christian milieu for the qualitative divide between what the church is and should be doing, and the all-too-often inherently corrupting nature of institutions with great law enforcing or law-manipulating power (i.e., corporations).  

If the government happens to be functioning properly and helping people, or even if the private sector can be harnessed to do this, it's seen as more of a necessary collaboration than a good one as such.  This is somewhat thanks to the influence of people like John Howard Yoder, Stanley Hauerwas, etc., and maybe post-liberalism in general (though I don't identify with it), but this might be too confined to the realm of academia, and I think a lot of it is just that culture, from the ground up, and in the communication age, has been changing like it always does.  Of course people tend to be ok with talking about the separation of church and state.  What we are far less aware of these days appears to me to be the need for separation of church and market.  I hope to talk about this in my next post.