[Maédhros]

What happened in between? Turgon became a founder, a builder, and a creator. He was Noldor, and the act of building his beautiful Gondolin awoke the fire of Feanor in his heart. He made the same mistake of placing his own sub-creation, glorious as it was, over all else, including the love he knew Ulmo bore him and his, and the utmost wellbeing of his people and kin. He risked them for a city, for architecture. He was honestly mistaken about the best course and the soundness of Ulmo's intelligence, but he also weighed the value of his city too heavily when he should have thought only of his people within it.

Truly, this is the kind of answer I was looking for. Great answer.

However, Turgon's mistake was much less serious than Feanor's and his sons'. It certainly led to a fatal error in his strategy for the defense of his people, but I don't think it rose to Feanor's level, selfishly denying the possibility that others also loved THEIR creations with a love as true as his own, including Eru and Arda, but also other sub-creators such as the Teleri with their white ships and Yavenna with the possibility of her healing the two trees.

I agree, but Feanor was a unique elf in the way that his works could rival those of the Valar themselves.

For Fëanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and in subtlety alike, of all the Children of Ilúvatar, and a bright flame was in him. The works of wonder for the glory of Arda that he might otherwise have wrought only Manwë might in some measure conceive.

I would say the mistake was Feanors and not of his sons. They didn't create anything like their father. They were just following him.