I'm pretty sure I've mentioned this before, but some of the best building advice I've ever received came from the man, the myth, the legend, Cajun:
They look good. I'd just beef them up a bit and fill them out. It looks like you're fond of using those little (++) parts. I'd get away from that and only use them if you have to. It'll challenge you more to find various parts that go together well.
Just a general thought - try to think of the pieces as shapes and not "foot" or "head". This'll help you get the shapes you're going for and possible get some out of the box building going.
Most of all, if you want to be a really good builder you have to build a lot. BUT you also must push yourself to build in new ways. IMO if you want to be good you should only sort of know how you're going to make something. If you're using this arm from model X and that leg from model Y and then just sticking a new mask on it you really are creating. You've got to push and then push again. I tell this to everyone but very few have actual done it.
I got this advice after I had so boldly asked him to review this:
(shown, really bad BBC entry)
I think that ended up paying off pretty well.
So really, If any of you up and coming awesome builder dudes want a review or something, drop me a link and point out what I should focus on.
Or not I guess.
But that part's up to you.