Comments about surveys or giveaways that promote pyramid schemes.
"Pay Per Click" referral links in comments.
Comments that falsely claim to offer full video content. This type of content could be:
Movies
TV shows
Concerts
Posting links to harmful software or phishing site in comments: "omg just got tons of B∪cks from here! - [xyz phishing site].com"
Comments with links to counterfeit stores.
"Hey, check out my channel/video here!” when the channel/video has nothing to do with the video it was posted in.
Posting the same comment repeatedly with a link to your channel.