One big thing that was done right was the setting of the objectives: defense of the indigenous Christians, rights of passage, and of Christian relics.
As the reason for the ultimate failure of the Chrusades, the lack of economic interest in the Palestine is usually cited. They didn't think much about crude oil then.
This anecdote (I am retelling it from memory, will check for accuracy later) illustrates the apathy that doomed the crusaders' Jerusalem Kingdom to a slow decline after the military phase had succeeded. Frederick in his travels discovered no less a relic than the King of Jerusalem crown virtually abandoned in a cathedral. Surrounded by his courtiers Frederick put the crown on his head himself. No one uttered a word. He, now the King of Jerusalem, left Palestine soon to attend to his politicking with the Pope. The fate of the Holy Land somehow ceased to matter to those people.
... On the insistent demand of the new pope, Gregory IX, [Holy Roman emperor] Frederick [the Second] embarked on a crusade (Sept., 1227), but fell ill, turned back, and was excommunicated.In 1228 he finally embarked. His “crusade,” actually a state visit, was a diplomatic victory. At Jaffa he made a treaty by which Jerusalem, Nazareth, and Bethlehem were surrendered to the Christians, with the Mosque of Omar being left to the Muslims. In 1229 he crowned himself king at Jerusalem. The pope denounced the treaty by Frederick, who was still under excommunication, and sent a papal army to invade Frederick's kingdom. Frederick returned in 1229 and signed (1230) the Treaty of San Germano, by which he was temporarily reconciled with the pope.
The eery coronation ceremony is retold by Brian Moynahan in The Faith.