Youtube
Youtube comments have both affordances and constrants.
An affordance is that you can freely write what you want to write. The constraint that comes with this is the trouble of trolling. People could insulting people through the comments on their videos.
It's also can be a very positive thing. If you write something that will make the person who's Youtube Channel it is, happy and positive, they will carry on doing what they're doing. They will make more videos for the viewers to enjoy.
If it's negative, they then will not make more videos and so you can't watch anymore.
It's also almost like a community. Everyone who positively comments about the channel, all have one thing in common: they all like the person's channel. You can comment queries and people will comment and support you.
Also there is a 'report' button. This is both an affordance and constraint. It's positive because if someone has commented an inappropriate comment then people can report them so they can't comment anymore. It's a constraint because people might go around reporting people who haven't done anything wrong.
You can also 'thumb up' or 'thumbs down' a comment. This will then show hoe good a comment has been rated and then result in to comment. This then enables everyone to see what most people think.