On the other hand, a world where everyone took a burn-the-heretics interpretation of Christianity or Islam 100% seriously would certainly have some advantages over ours, and especially over our middle ages- things like no un-sanctioned killing, most notably, no wars against others of the same religion, etc. Probably lots of things that would be decent ideas if you could get everyone to follow them, at the cost of an occasional burnt heretic (and possibly constant holy wars, until one religion gains the upper hand and overwhelms the others).

Sounds like the history of Europe and the Islamic world. Except that no-one ever did get the upper hand, neither for Christianity vs. Islam, nor the splits within those faiths.

Anyone want to go back to the time of the Crusades?

Probably lots of things that would be decent ideas if you could get everyone to follow them

If the only thing in favour of an idea is how wonderful the world would be if everyone followed it, it's a bad idea.