Okay, let's see.....
In FS, most information about the game, the ships and weapons, and other gameplay options, is contained in simple text files. Apart from a few instances where binary files are necessary (models, images and sounds), everything there is to know about a mod can be discovered by reading the table files.
Of course, this means that modifying said data is rather easy, as you can use a text editor to do it.
Now, headdie said that this can be inefficient. He's wrong. Parsing this information is a one time operation that you have to do on startup anyway, and I would say that error-checking a human-readable file is easier than hunting for errors in a binary.
Other games, however, store the same information in binary files that require specialized editing tools to read and edit, which makes modding extra hard if those tools aren't available.