Anything that happens after choosing sides in the FW story can be considered spoilers. This is a very broad definition though.
The following are considered spoilers.
- The Free Worlds story, after choosing sides.
- Quarg presence outside of human space.
- The Korath.
- The Hai and their missions.
- The Korath Automata.
- The Wanderers and their story.
- The Remnant and their missions.
- The Coalition and their missions.
- The Ka'het
- The Drak.
- Anything related to the Pug.
Mod content can also be considered spoiler, though this would be decided at the creator's or subreddit mods' discretion.