There is necessarily some overlap to these plagues
1) AOL (1990's Internet gateway confused with the actual Internet)
4) Penguinistas (from the advent of Linux until Linux became a stable and mature OS, c. 2004/6; subsequently, for the most part, it's all good, ignoring the systemd pimple)
4) Adobe Flash
6) Apple and the development and ubiquity of the iOS-dominated mobile web (this ruined nearly everything for mobile device power-users)
7) unrestrained web developers and site feature creep, KISS is replaced with incomprehensible complexity (slow steady march to WWW apocalypse)
WWW/Internet never needed any of these things. Some of them started out innocuously enough, and turned evil (like Flash), and some started out evil and turned to goodness (such as Linux and it's irrational popularity prior to become mature and stable).
What will be the next scourge of the Internet?