Certainly there are many applications that demonstrate the need for high-performance server APIs. FastCGI and the Oracle Web Request Broker are two proposals that address the need for a high-performance server API. I won't beat this one to death.
An issue that was raised repeatedly during the workshop was that HTTP, as practiced, is not a good interface to layer applications upon. The root of the problem is the lack of separation between application data and user presentation issues. A typical server application fetches application data and then merges it with a screen template to create an HTML document. An application must parse this HTML document to recover the application data. This is not reliable.
What's wanted is for the application to generate the data and screen template as separate MIME parts. Then either the Web server or the browser (or neither) can perform the merging, depending upon the needs of the client and the capabilities of each.
Achieving this separation of data from presentation will have a huge impact on the economics of developing and maintaining Web applications and on the ability to compose large Web applications from smaller ones. As such it is extremely important. I believe that the Tcl/Tk group at Sun Labs is working on an approach that achieves this separation; surely others are as well.