Probably been requested before, or even already implemented but I can't find it.

I'd like to have the option to add tags to cards.
I would use that feature to add tags like 'removal', 'mana ramp', 'card draw', 'graveyard hate' etc.
That way I'd be able to identify similar cards and choose the best ones and throw others away. I have too many cards...

Im wondering that too
WOUld love to tag my legendaries

This would be really useful as well, to be able to have custom tags

I was hoping to use this to tag where my cards are (in binders, in decks, borrowed by friend X, etc)

I would love this feature as well. I would like to import my MTGO cards but I need a way to segregate them and tags would be the ideal mechanism.