What size did you make the planet at , te texture seems huge. I mean.. how many features are there that stick out THAT much. ( I mean the lines are massive, on the texture map ) Also, when you do a shadow, make the shadow size Bigger then the planet, THEN fit it into the corner or w/e, it'll make the lightsource look more realistic. Try to add a glow instead of making your texture bigger then the actual planet, it just looks kinda messy on the edges. And as for realism.. then it kinda doesn't work....

Basically
-Use photographs and blend them to get more realistic textures
-Work at a stupidely high resolution, so the details you add look realistic and small
-Work ALOT on the colouring, use tiny brushes, colour tiny areas completely different shades. And mess around with layer settings etc.
-Make the shadow bigger then the planet, then move it around. And then cut off the excess.

I can't think of anything else atm... listening to Dj yoda =)