Deckbuilder still needs work. I am hoping they will add one single custom filter that lets you put cards into favorites or equivalently remove cards. With 4000 cards, lot of reprints and draft chaff in the collection it is about time.
There's some more stuff we have planned for the deckbuilder, but favoriting in the sense you are talking about isn't currently on the roadmap. I'm interested to hear more about how people would want to use that sort of feature though, so I know where the current pain points are.