Ground plane under the crystal can be a problem.
You need to add a 0.1uF capacitor between Avcc and ground.
I'm not sure whether C6 (between Vref and ground) is whrthwhile, the datasheet doesn't mention putting a capacitor there, although I can see that it might help filter out power supply noise.
PS - you should join up the ground plane traces much more. You seem to be trying to avoid having lops in the ground plane, but there is no need to do that. Aim to put ground plane almost everywhere there isn't anything else.
You Want ground plane under the cap, try for both sides if possible. See attached.
On a side note, you need a good place to get these manufactured! I used seeedstudio for mine, and the boards came out pretty decent, especially for the price (although silkscreen wasn't amazing).