val'Holryn brings up a great point. Blood Reign of Nishanpur gives the story of how Manetas became fallen. Sarish watched as the Valinor of Illiir almost crushed the Valinor of Neroth in a heavenly battle. Fearing he would be next. Sarish send Loshnek to turn the tides. Working against Manetas's pride, he told him he's never be more than the first among the God's slaves (Valinor). This caused Manetas to revolt against the Illiir's wishes, leading him to the story in the Blessed Lands book, and taking 1/3 of the entire Valinor host with him. but in the end he failed. The Valinor that sided with Manetas were also cast out. Sarish taking advantage again, gave each Valinor a choice, he couldn't undue the others' commands, but he offered them the chance to lead his Infernal forces against the Gods' enemies and maybe one day they would atone for their mistakes. Urumeh the Honor of Cadic, and many others, took the chance and we saw him leading a losing battle on another plane. I can't find the reference but the ones who didn't take his offer were cast out to wander the void. (I'm guessing that means wander space.) Fast forward to the End of Year 6. Manetas is back, his new and improved plan is in action. We learn from the Love of Illiir, that again the Host is divided (not equally this time) those against Manetas (the smallest group), those with Manetas, and those who choose not to take sides (the largest group). Thanks to magical coercion those who choose not to take sides join the group against Manetas. Once Manetas was again trapped, all the Valinor that joined him were trapped with him as well.
Pride became too prideful, Patience lost his patience, Honor lost his honor. These are the only 3 fallen Valinor we know of the rest are speculation. As for the difference between fallen and not fallen, it would appear to be power. In the story of how Patience lost his patients Xabal states.
Quote:
His role was only to observe, never to act. But he had acted, had done that which could not be undone, and whatever happened, he would never again be the Patience of Sarish. Even now, his master would know what had been done, and would be stripping all power from the Valinor that dared break his rules – the rules of the Binder, that none dared defy. ... With that word, the last of Sarish’s power was truly gone from the Valinor, and he fell to earth.
What power? Could we break it down into the mechanics of the game in a way we could understand, possibly. The end result is when a Valinor goes against the final order from the Gods (like Patience), or when their God declares it (like Manetas, or Urumeh), they become fallen and lose something.