I don't think a file manager is a great example here, because it's expected to be something of a jack of all trades. But right now one of my favorite applications is [link|http://basecamphq.com/|Basecamp], a web-based project-management system with an almost absurdly small number of configuration options. The developers started with a single thesis: project management is not about graphs or charts or four levels of committees signing off before something can go forward. It's about communication. So they built an application which made communication simple and easy, and kept things tightly focused. At first they only used it in-house for their web-design clients, but then they began marketing it as a service and now are making boatloads of money from it. And its focus hasn't hindered use one bit -- plenty of web designers use it for project managemnet, yes, but it turned out that "project management" is a pretty broad thing, and now they've got customers using it for everything from major corporate processes to wedding planning.
\r\n\r\nSo "focus" doesn't mean "define exactly the set of things users will be allowed to do, and forbid all other uses".