The abilities.txt file is just a listing of the available abilities in the game (although not all of them work).
Fyron's modding tutorial has a list of all of the abilties and which files they work in:
http://se4modding.spaceempires.net/ModdingTutorial.html
The formatting itself is not really a problem (because it's copy/paste or automated) and the error reporting by SE:IV when loading a mod catches missing ability fields, but not extra ones. It also does not pick up on unused levels etc. - although you can usually find these by using the internal tech tree report. But a data file check would be cool.
DavidG made a data file editor for SE:IV that works fairly well with most of the files:
http://www.captainkwok.net/files/se4modder.zip