I don't know if I'd ever want people to comment on cards or streams I was thinking today that if your browsing a stream and you have a thought that you'd like to add, why not have the ability to suggest a card to add to a stream?
The use model would be click on the suggest icon for the card and either select an existing card you may have from an existing stream for repost in the stream, or create a card right there. The next time the user of the stream logs in, they would see something like "Stream foo has n suggestions".
You review them, and post the cards you want into your stream and decline the ones that you don't want.
I think suggesting cards to add rather than allowing comments, encourages people to offer something more constructive to the stream rather than just "lol", "great thought, I totally agree." etc.
To me it seems like an interesting way for users to contribute to the stream and interact with one another while allowing the owner to moderate the things that go into it. It also does not pollute the site with a load of mindless comments.