FriendInFlatbush, succah on Shmini Atzeres is halachah.
My list:
Halachah: not drinking pig’s milk
Minhag: waiting six hours between meat and milk (for those who follow R’ma)
Chumrah (a tough one, because one man’s chumrah is another’s halachah): making kiddush only on unsweetened wine
Shtus: calling pba’s posts shtus