I really don't think it gets any more casual than Rolling Stones. It makes a logical deck, that can be mildly effective, but isn't really any good on a tournament level. Also allows you to play a bunch of other cards that would normally just sit and rot in your binder/box.
I will nominate Ensnaring Bridge, since a lot of casual decks used this, but most of them used it badly and locked the game down, including themselves.
So in multi-player games, they now wore a target on their forehead.
All the while the player that played the bridge would complain about not being a threat, yet everyone was targeting them for death.......