The first thing that comes to my mind is more of a philosophy rather than a feature. One of the Agile principles is don't implement things until you need them... no speculative development: the tenth principle down in http://agilemanifesto.org/principles.html reads "Simplicity--the art of maximizing the amount
of work not done--is essential." The XP process implements this with its rule that you only write code to implement the current iteration, no writing code to implement features that you "know" will be coming in future iterations.
When I first read the Arc web server code (srv.arc) I was astonished at how simple the code was.
srv.arc doesn't do everything I want, but other web frameworks don't do everything I want either. And I've found it much easier to hack srv.arc to do what I want, precisely because it isn't massively bloated with "framework" stuff, stuff that we imagine we'd need to write a web application, but we don't actually need or it turns out we need in some other way when we actually write an application.
Showing off how easy srv.arc is to hack is a great example of the Agile simplicity principle. Need to get a servlet to output a custom header? Plow through the documentation to find the right addHeader() invocation and what object to call it on. Need to get Arc to output a custom header? (prn "My-header: foo")
This is interesting, I had been thinking only of how the core language might appeal to an agile mindset but it's true that the libraries have a lot to offer as well.
