DMGTwo Problems
Two points. First, a variation on JWZ's famous quote:
Some people, when confronted with a problem, think "I know, I'll create a custom scripting language." Now they have two problems.
Second, remember that config files are just programs written in simplified language. So:
Some people, when confronted with a problem, think "I know, I'll put this in a config file." Now they have two problems.
My rule of thumb: config files should contain the information your program needs. It should not affect the behavior of your program in any but the simplest sense. When I hear that something is "extremely configurable", I get a cold knot in my gut, and ask to see the config file. Then I start asking about syntax, error handling, includes, re-use, deployment, version control... all the things that are necessary for maintaining code. Because config files are code.