The leisure=stadium tag implies building=yes/stadium/multi_storey/.... Otherwise, the thing is a pitch.

Don't believe me? The Memorial Coliseum and the Rose Garden Arena are both stadiums. As of 12-mar-2010, someone has decided that the Rose Garden Arena also needs an explicit building= tag. See the difference yourself.

osm.org/?lat=45.531731&lon=-122.667919&zoom=18&layers=B000FTF

Interestingly, there are some facilities that need the building= tag to render. For example, a bus station renders but a train station won't. Compare Union Station to Grayhound. (Well, or you could but the railway=station tag has been removed from Union Station, sigh.)

osm.org/?lat=45.528022&lon=-122.676062&zoom=18&layers=B000FTF