[summerlight]

I think it's also very important to see (and easy to miss) that a language is just a part of bigger programming systems. Every problem domains with enough complexity will eventually grow into some sort of a domain specific programming system to provide flexibility and simplicity. Composition, abstraction and generalization are all important tools to tackle this problem and programming languages exist exactly for this reason.

Because general purpose language only solves a tip of iceberg, you will find out that your system begins to resemble more and more of a language as your system evolves. (Although it's usually best to avoid this situation IMHO, but if more than thousands of users depend on it then it's inevitable!) At this moment, some knowledge on language design and implementation could be helpful to avoid pitfalls and make the system more consistent and easy to the users even if it doesn't take a form of a textual language with its own unique syntax.