When a card gets a comment I get the indicators that there is something new there, which is nice. But sometimes I can't respond to the comment immediately, so it would be nice to have the option to "mark unread".

Pretty much same way that email and Slack messages have. Basically it's so that I don't forget to get back to it the next day.

Of course some other feature could handle the same need, like some kind of bookmark/save for later or "remind me" etc. But I guess just mark as unread would be simplest one to implement.