What Ideas Deserve Words?

At "English Language and Usage Stack Exchange", one common kind of question is "Is there a word or phrase that means . . . ."

Sometimes there is one in English, and sometimes there isn't.

What does one call the cognitive ability involved in being good at coming up with such words?

What makes a concept worthy of having its own word? 

Also, what makes a concept worthy of having its own "simple word" as opposed to one that strings lots of other prefixes, roots and suffixes together to form a "complex word"? Where do these words, especially the "simple words" come from?

How do people in linguistic communities make decisions like these in a language community like English or particular English dialects that doesn't have the equivalent of the French Academy to render official and formal judgments about what words are and are not correct.