"Unique snowflake" was used in a positive sense before the movie. The novel uses it, but Durden uses it in a disparaging way. He uses it to mean anyone who believes themselves above his orders, because it's feminine to not follow his orders. If you think you're a unique personality, you're actually not, because feminine society brainwashed you with empty comforts. The opposite of a snowflake is what Durden calls a "space monkey." The ones who are willing to follow his orders even if he's not making sense or the orders seem dangerous.

The entire book portrays feminine things as invalid, or weak. Bob has breasts because he's feminine. Working at an office is feminine. Worrying about how you look is feminine. Masculinity is gained through being a terrorist and hanging off the every word of the big manly guy who punched everyone.

Chuds read the book (let's be real they watched the movie) and didn't realize the message was satire. I blame David Fincher for casting the attractive Brad Pitt as the villain.