All gold is account bound, as are all crafting materials (except Exquisite Essence, Iridescent Tear, and Fiery Brimstone) , quest items, and marquise and higher gems. Gems below marquise can still be traded.
All legendary and set items dropped since patch 2.0 are account bound, to an extent. The item will be tradeable for the next two hours of gameplay to any player that was in the same game when the item was dropped. After two hours of gameplay, the item will be permanently bound to the account in possession of it. Crafted legendary and set items are account bound as soon as they are crafted, regardless of the presence of other players.