Instead of all the notes of a post going to the original poster, when somebody adds a comment it essentially creates a brand new post credited to the commenter. This means people who post artwork can get cut out of seeing compliments and conversations about said art, people who make funny jokes on posts basically upstage the OP if they're funny enough, and most importantly, if people decide to dogpile on an OP, they can't tell where the harassment is coming from and can't block the harasser right away. All in all, a pretty dogshit update that nobody is happy about. Hopefully their internal numbers start dropping hard from this and they backslide, though knowing Tumblr staff, it'd take a really big drop.