Nathan Dowdell (lead rules developer for Modiphius’ Star Trek) made a distinction on rpg.net between necessary and desirable house rules —
- Necessary rules fix problems with the game. He needs to make the game playable and enjoyable for him. Making these is a pain; making these is work.
- Desirable rules take a good game and make it better. They extend the game, adding new options or adding detail in areas he’s interested in. Making these is fun; it’s a leisure activity.
I see an analogy here to Zak Smith’s distinction between Fat and Skinny games — fat games have extensive library content, while skinny games just have the rules. Fat is easy to cull, easy to add; skin is not. Changing skin is hard work. Change the skin wrong, and all the fat falls out, and stains the carpet.