If you have suggestions for specific datasets, cards, or dashboards, your best bet is going to be emailing the person who created them. If you don't have access to email and the dataset or card is an "official" one, you might also be able to use mySupport. It'd be nice if Greenfield had a more direct method of offering feedback though.
I'm kind of curious which cards and datasets you're talking about,
@PogDog.