You'd add a moonkin for Earth & Moon, which is 13% spell damage which is a buff the shammies do not have.

However, for the druid to do comparable damage to a shaman, they will need to manage their Eclipse procs, which means spam Wrath until you get a proc and then Starfire until you get a proc. Ideally, your rotation is Wrath or Starfire on trash, and Insect Swarm > Moonfire > Wrath/Starfire on bosses. You can also get 3% hit from improved faerie fire, which is significant in raids, but likely not going to do much for you in heroics or dailies.

If you want the 13% without managing anything, go with a Warlock, any spec. Curse of the Elements will do the same.