Honestly the black parts look unfinished, like they were just undercoated and not painted. If they are intended to be black, then highlight them in dark gray.
The helmet especially needs the details picked out. There's all sorts of little bits on the helmet to pick out and make it the focal point of the model (second maybe to the enormous phallic symbol that is his bolter
). Most importantly though are his eyes. Marines without painted eyes seem lifeless. When I paint my marines eyes (which are bring blue with a slight glowing light effect), it seems to imbue them with a soul, and they look finished. Go with something contrasting, like orange, yellow, red, or something similar.
The blue armor could use a wash and a highlight, both on the marine and the rhino. Some of the metals could use a wash too, but you seem to have made them reasonably smooth, unlike the globby mess others sometimes produce.
And the marine needs to be based. The more I look at this, are these guys WIP? It just didn't seem like if from your description. Anyways, you've done a decent job so far. The best quality I see is that the paint is even and not clumpy, which leaves you free to add all sorts of nice details.
