Great skill!
I'm going to go over some of these, since I think that composition wise there is some stuff left to do.
#1: Promo screen:
What is lacking imo is depth. The rock on the foreground needs to be detailed more and lightened up, so it comes out in the foreground. Try making it a blue that's the opposite of the reds used in the background. A glow around the parchment to make that jump out would be ace too. As for the parchment itself, I don't like the black edge on the curl. Symmetry could be broken by putting the foreground rock to the left and the city to the right.
#2: Just lovely scene, breath taking, no crits for it...
#3: The faces could use some extra highlight, or a secondary light source, perhaps even some sort of ambient lighting. Also, you need extra highlights on the glass and the helmets, those look very dull.
http://www.spriteart.com/tutorials/02_ambient_face.html <- might come in handy...
#4: I don't like all those black outlines, try colouring them depending on where the light comes from. This is alot of work on this ammount of tiles, so perhaps you'd better keep it in mind for next project or something

. Also, is that lava? I dunno if you looked at real lava, but it's firey and bright with ALOT of contrast, I'm really missing that here...
#5: Pretty good, but I'm missing contrast, and perhaps a hue shift...