Here is what i think of your scenes:
1-(piggy): The reflection looks kind of wierd...and did you use a tutorial to create it or you did it on your own?
2-(bomb): it looks good, but the material isnt that good
3-(explosion): it looks really nice but unatural, i mean search on google for nuke bomb and compare your scene with it 😛
4-(castle and dragon): the modelling of the caslte is very good, although lightning and textured are not very nice...
5-(cabin): the table and fire place lack alot of detail, and the texture of the table and chair are not natural, and the scene is too blocky.
6-(earth): i have never seen such an image of earth, the texture is too big, or maybe fake, and note that in all earth images, there are always two side, one which is lit by sun light, and the other which is dark ...because there is no sun light, oh right, there should be some clouds around the planet. And concerning the moon it seems as if it is glowing.
If you felt that i am saying that your scenes are not good, please don't get me wrong, i am just telling you how to improve...and I'm not a pro you know, so there may be some people out there with more experience than me that could help you out more.