The Valar sin is actually pretty simple and exactly what they rightly Féanor for. They hoarded Light and did not use it for the benefit of the world at large. I would also add Turgon's fault: They loved too much the works of their hearts and hands and refused to leave them.
When Melkor destroyed the Lamps, their solution was to build their fortified paradise rather than fight him (Tulkas was already there so they could have won). When the Elves awoke, rather than leaving their fortified havens and dwelling among them, they found more convenient to lead them to Valinor, forsaking those who refused to the tender mercies of Sauron.
And what was the results of that? When Men awoke rather than finding elves to teach them, they found Morgoth, who was the only Valar present on mortal Arda.
Even after the Murder of the Trees, their solution was to fortify further rather than sally out. Whatever the Noldor had done, abandonning them and the Sindar who had done nothing in this mess to fight the most powerful being to have ever walked incarnate in Arda was a dick move.
