I'll add one more to this, "Pleasant personality". If you have are difficult to be with, you will be miserable and so will others around you, leaving you with very little or no allies. So new formula:
Valued = Useful + hard to find + easy to work with
"Easy to work with" is a wide requirement. It's harder to find.
But we can compare how far "easy to work with" goes in a more balanced scenario:
You're hiring for a critical rare skill. _Only two_ candidates were seriously considered, and you hired them both.
One of them delivers for the skill you hired (useful), but is not easy to work with.
One of them does not deliver for the skill you hired (useless), but is easy to work with.
So, it doesn't matter what your definition of "easy to work with" is in this scenario. It justs matters that whatever it is, it's not related to usefulness (so you can't get away with "but being useful makes you easy to work with").
Who would you lay off if your bills were depending on those guys?
Valued = Useful + hard to find + easy to work with