This is a hard game to rate, considering that it is up to the players to make missions. A player could make an inappropriate mission or creature. Maxis can ban a player for this, but with so many creations, some may slip through.
Most missions involve violence, but little other questionable contet. I have encountered one use of a**, but it was by a brazilian, which may be a lighter cuss-word there, but I don't know. A strange deal was made for this; Robot Chicken missions. Why Maxis would have Robot Chicken make missions I don't know, but they are seperately rated (T). The missions are also downloaded optionally.