Yes, your biggest problem is definately length. You're pretty good at melody, but it either gets repetitive, or doesn't evolve enough to be satisfying. I think you need to experiment more with theme and variation.
I like your instrumentation choices, too. It's not really anything special, but it makes sense.
Nice remixes, btw.
Anyway, I've pretty much told you what I think of most of them. Work on extending your songs though, and you'll be doing great.