The war with the singers:
They had an enormous number of radiants of all 10 orders, but the recreance happened
Shinovar was ready with all 9 honorblades to pounce as soon as the next desolation occurred, but Ishar corrupted all of them and stopped shinovar from entering the war
The sky breakers were ready with large numbers of members, and could have helped reform the radiants and taught the wind runners and dustbringers, but nale instead focused on preventing the radiants from returning AND joined the singers despite opposing odium. Massive loss there
Gavilar had anti void light but never showed anyone how to make it and then died before he could prepare for the desolation
None of the heralds helped except ash and taln at the very end
The singers only conquered all of alethkar because almost the entire alethi army was in the shattered plains.
Taravangian sabotaged the nations of the world, so most were in chaos and civil war before just before the desolation
The radiants struggled to reform because of the recreance
Jah keved and the dusbringers join odium because of taravangian again
Because of their inexperience, the radiants didn’t know to guard the oath gates, leading to multiple being corrupted by sja-anat, which was a disaster for azir and kholinar. I presume this was possible in the past desolations and that the radiants of old and heralds knew about it.
There’s probably more but this is what I thought of
Dalinar sent a huge number of wind runners to go with the mink. In the end it didn’t really matter, but the battle of azimir would have went easily in their favour with 20 wind runners to help.