There's nothing I like more than a surprise lion. This card is great because, in most situations, it'll be free. You just have to wait until that pesky Aesthir Glider swings at you and, BANG! There's a cat in the way. Other than that, it's a 2/3 with reach for 3 which isn't terrible. Not great, but not terrible either. The free clause is what makes this card good. Yes, a creature does have to be attacking you and you do have to control a Forest and a Plains but those conditions aren't exactly hard to meet. In a dual-colour deck you should have both basic land types on turn 2 (or turn 1 if you're running fancy lands) and your opponent will probably swing at you at least once during the course of the game so this card will probably end up being free.
Pros: Can easily be free
Cons: Not a superb creature, conditions have to be met for it to be free