-- lifts a quality out of the thing that has it and names it. gives , gives , gives o, gives o, gives o. The result is a noun in -o, and eco stands alone meaning a property or characteristic.
The contrast to keep straight is with --, which names the concrete thing instead: is beauty, o is a beautiful object you could point at. Confuse the two and you compliment the vase when you meant to praise the idea.
One practical note. Plenty of adjectives already have a plain noun sitting beside them, and the plain noun usually wins. Varmo is heat and o is warmth-as-a-property, which is rarely what you want; reach for -- when no simpler noun exists.