Ok, ill tell you about the last one: you need to work on your shading and 3d light, its shaded in stripes, instead of in curves:
Its most obvious on the last one, and you shaded best on the second last one. Just try to watch out for that, and make edges more obvious through heavy lighting and shading.
Your learning pretty fast i think, but you could learn faster by just studying a single object or part closely. For example, grab a die, and put a bright light diretly on it from a sharp angle. Now position yourself so you can see the top and 2 other sides, and then draw it in high resolution (1024x1024 at least). Try to work on the shading a lot, and just make it look as good as you can.
This is the best way to learn i think, lots of smaller high detailed pieces.
Good job so far man, keep working!