A bigger feature missing in most GUIs is that I can't tell what has changed from the defaults. On the other hand, most text configuration files contain only what has changed from default, which is exactly what I want to know.
I would say the biggest problem with text configuration files is that you can't ever be sure what the default is. The problem is that: 1. Config files need _visible_ defaults 2. Config files need to show current values 3. _Config files need comments_ 4. Config files need to be small or they get unreadable Doing all four is tricky. Postfix gets close: postconf will show you all config entries. Add -d to get defaults i…
Standard convention for a lot of software is to have one config file (for instance often with the extension .dist) with all the default settings, and lots of comments about them, and a local config which is the one you edit.