customizable feeds you can subscribe to + starterpacks instantly give you full timelines and people to follow (and followers, if you're in many starter packs)
better discoverability, and therefore higher engagement
stacking moderation and excellent security features (e.g. detachable quote boosts, "the nuclear block")
many users who tried Mastodon first had bad experiences with "HOA"-like behavior and over-enthusiastic mods
Home Owners Association a group or people that "polices" neighbors and has a hisyory of doing shady things. But he's referring to the actitude of "coming outta nowhere to tell you what to do" they have in common.
What S_H_K said, people have reported being rebuked for posting pictures without ALT-text and not CW-ing uncommon things like eye-contact or food, for example. One person notably received angry messages for posting about cutting their finger on a sheet of paper without CW. The worst accounts were of POC talking about racism they experienced and being told to put it under CW.