Updated. The activated ability now marks only creatures instead of any card.
Note that the poison effect will also count marked permanents as well, but you will need another card to mark those. This should help promote synergy with other cards in the series.
It will also allow creatureless decks to work as counters to this card (particularly if they are able to CC enemy marked creatures as well).
... I suppose that makes this change both a buff and a slight nerf at the same time...