You have to have the sweet potatoes baked with the little marshmellows on top. It's almost cliche.
I grew up in Missouri, so I'm not really a southerner or a yankee, but our family ALWAYS served two meats, no matter what the holiday. Quite often, two or more families showing up at grandma's house at the same time constituted a holiday. We also liked cheese grits drizzled in honey for holiday meals (I know! And I still say I'm not really a southerner). The other big Thanksgiving tradition is pies. Any pie will do, but most especially pies made from fall produce--apple, pumpkin (with copious amounts of whipped cream) and we usually had minced meat pie, too.