I'm not sure if this has been posted but it is not on this particular thread: here is TGC folks explaining why they think Complementarianism is a big deal for the organization -
http://vimeo.com/44525286
The issue isn't "why does the Gospel coalition hold to a complementarian view", but rather, stated by Carl Trueman, "Why, for instance, is this issue of more importance than, say, differences over baptism or understandings of the Lord's Supper?" I think you miss the point. Here is the main point Trueman makes:
"This is not the only awkward question one might ask: for example, which is more unacceptable to a Baptist - a woman preaching credobaptism or a man preaching paedobaptism? But that is for another day. In the meantime, do not misunderstand me: I do write as a convinced complementarian and a member of a church where no elders or deacons are - or can be -- women, though none of them are - or can be - Lutherans, Baptists or Dispensationalists either. It is thus not complementarianism in itself to which I object; I am simply not sure why it is such a big issue in organisations whose stated purpose is basic co-operation for the propagation of the gospel and where other matters of more historic, theological and ecclesiastical moment are routinely set aside. If you want simply to unite around the gospel, then why not simply unite around the gospel? Because as soon as you decide that issues such as baptism are not part of your centre-bounded set but complementarianism is, you will find yourself vulnerable to criticism -- from both right and left -- that you are allowing a little bit of the culture war or your own pet concerns and tastes to intrude into what you deem to be the most basic biblical priorities."