2 new ones.

[Chidori.]

Heres 2 new ones.

[tunafish69]

The TimeShift one has too much blending, try getting rid of some of it. Try blurring the C4D's in the background some as well. It needs a LOT of depth. Your focal point is a little over-sharpened. Your light source is like a solid block, try using a 200px soft brush on about 20-30% opacity and click 1 or 2 times where you want your light source to be.

[Ptka]

Both are a bit over-contrasted, maybe make there be less light because it's a bit bright. Move the text away from the corner on the first one and change the font to a sans-serif and a bit smaller. No gradient map on it.
For the second, the text is good because it fits with the render.
Get some more depth by getting a stock background and blurring that, and sharpening the render. Read some tuts, from Papa, Garis, Immortal, Soggy, Elramo, and others. :P
Good luck with learning!