I wish lists let me define custom fields in the context of a ListItem Group.
Doing that, one could extend lists to cover a ton of different scenarios.
Lets say I have a list called Cars
So in that list each item would have a custom field defined for each thing that describes a car
Then you create a new list called Planes, and like above, that Planes List contains a bunch of properties which apply only to that particular list.
I was a bit disappointed to see that when I created a list item in Cars, it shows up in Planes as well (as it didn't apply)
Kind of makes the custom list fields super useless to me as I can't think of very much overlap between what I'd want to be in a list.