Couldn't get better than this. Wenlong has pretty much detailed out to the gory details of understanding WCF scalability in these posts.

1. WCF Request Throttling and Server Scalability
2. Orcas SP1 Improvement: Asynchronous WCF HTTP Module/Handler for IIS7 for Better Server Scalability

The fact you realize is that one bottle neck is enough to kill your scalability.