There are numerous things frequently found online that are inhumane, evil, wrong, and just plain annoying. Here are a few that come to mind.

Gallery views of trivial lists of trivial things. I don't want to wait 8 seconds for a mediocre "revelation" to load 25 times. If you use a gallery, your content should be nothing short of revolutionary.
Popovers where I can't see your page without clicking 'Close' first. See: Tab Closed; Didn't Read.
Multipage articles. Unless it is John Siracusa's epic saga outlining every detail of Mavericks, there is no reason the content should span more than one page.
Auto-play content. Partial redemption if it is muted by default, but not much. If your site auto-plays audio or video then you may as well just have your domain forward to your MySpace page. Make sure to leave your pager number on the Contact page.
Persistent prompts that ask you to take a survey, if you want to live chat, or anything similar. If I scrolled down and didn't click on your abomination, then there is a very good reason.
Mouse-over ads. Going to a website shouldn't be a tedious minefield while scrolling down hoping to avoid the double underlined words that popup ads when your mouse touches them.

Any suggestions? I wouldn't be surprised if this became a regular segment.