5. The Factionless
Absolutely nonsensical. Why would you discard possible man-power? Especially when you seem to need every single person you can get?
I would understand if the factionless where made into slaves or something - but they're just... there! Why? When they fail in one sector, why aren't they given the chance to succeed in another to help a community? You make the choice at like 16, for God's sake [worst time to make life-changing decisions, by the way], how can you possibly be expected to make an accurate one? It doesn't make sense that people - at any point in time - would just discard a population. They need every hand and every foot they can get.
The only example of this happening - ever - that I can actually recall, would be the Sudra or untouchable caste in the Hindu Caste System, but even they had street-cleaning and house-serving roles.
But the whole plotline falls if the factionless don't exist. The Abnegation wouldn't be withholding resources to help them, the Erudite wouldn't be railing and then killing the Abnegation for it by mind-controlling the Dauntless.
The whole story stands [and falls] on a significant flaw!