And I agree with your stated differences. Western Civilisation has indeed changed since medieval times. Nnot necessarily for the better, either, as it cannot any longer be called "Christendom". I also agree that this present darkness is adequately covered on other threads. I would suggest, however, that study of the medieval Crusades might reasonably be approached from the "lessons (to be) learned" point of view. What was done right then? What was done wrong then? These questions can be moral, strategic, tactical, other, or some combination. In so far as we are still fighting the same enemy, nearly a millenium later, such lessons could be highly instructive. However they should, as you requested, be addressed within the context of their time .
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.