This page has a full list of differences: https://join.piefed.social/features/
tl;dr is that:
- It's written in a different language, so appeals to different developpers, also claims to be easier to maintain and more efficient, but I can't judge that.
- Has Post & User Flairs
- Has emoji reactions
- Has polls
- Has Post & User Flairs
- Has multicommunities
- A chat feature
- Better moderation
- a more feature-rich and (subjective) nicer default UI with stuff like consolidated comments,
I can access piefed communities and posts from my lemmy instance: PieFed link: https://piefed.zip/c/fedilytics@piefed.zip/p/1963727/growth-of-the-fediverse-20261102 Lemmy link: https://lemmy.zip/post/74312955
Most popular communities are lemmy ones, so it might just be that you haven't seen one yet, they should be perfectly cross-compatible (minus the extra features), since they're both threadiverse software.