You have to get all golden flagpoles. I almost had that completed, but then I had to format my 3DS and lost my save...Anyway, just spent the last couple hours trying to do a 100% Super Mario 3D Land run. I beat every level with all 3 star coins and both brothers (in both "types" of worlds, if you get my drift), but I still didn't get the fifth star or the "reward" that comes at the end. I wonder if you have to get all golden flagpoles and star coins within the special boxes hidden in levels... anyone else know?