We looked for comprehensive documentation and, for the most part, were not disappointed. Microsoft includes excellent HTML documentation but no hard copy. As always, O'Reilly's print manual and HTML help were impeccable. Apache provides almost no documentation other than some READ.ME files. Once your server is running, you'll want to fine-tune it.
Some packages allow more advanced customization than others.
Having access to the source code in Apache provides the ultimate in customization. More common is the ability to create custom error messages and default Web page headers and footers, something Enterprise, ICSS, and Web Commander could do. The More Things Change . . . The dynamic nature of Web content means Web servers need a fair amount of hands-on attention. Thanks to Web browsers, you don't always have to be in front of your server to administer it.