1) Way too much email. There is no weekly digest so you are either going to get daily emails for the different categories/types of notifications or none at all. I got sick of all the email and unsubscribed.
2) It really started to depress me seeing all the racist posts and petty complaints people in my neighborhood were posting. Up until then I thought of most of my neighbors as nice people. After seeing all the "suspicious black teenager seen on X street" posts and other awful petty stuff I started to feel like I would rather not know what they think. Basically Nextdoor makes me feel like I live in a neighborhood of racist, bullying busybodies who have way too much time on their hands.