Mana base sucks - went with RG lands instead of UG and UR lands because they are less expensive.  Invisible Stalker is there to replace Tarmogoyf - not because it's similar, but because it's good.  Would like to see Quirion Dryad in this deck - kind of a gro/delver hybrid, maybe.  I don't think nimble mongoose is as good in casual as it may be in legacy.  Those high-powered, fast decks put cards in the graveyard fast.  It may be up to you to fill the graveyard in casual, so it could be a while before the mongoose is 3/3.  If you can't fix the mana base cheaply, consider two colors.  Red and Green are both good splashes, but splashing isn't so easy with few duals.