- Tracking the user's internet behavior. - Hoarding the user's private information. - Getting in the way of content. - Autoplaying audio or video. - Coin mining or other malicious scripts. - Outright lying: click to claim your prize! Your pc might have malware so download our toolbar! - Misdirecting: fake 'Download Here' buttons or 'Press X to close'. Worst offender I've seen was a mobile ad that had a fake smudge on your screen, tricking you into tapping it. - Showing NSFW ads on an otherwise SFW page. - Hogging way more bandwidth (and loading time) than the actual content.
This should be pinned under the search bar on google's starting page.