The first thing that strikes me is that people posting projects don't really understand the value of software engineering. They want someone to do crazy ambitious projects for pretty much no money. As if to say, "I have this idea which is really awesome and valuable, now I just need some monkey to do all the work." $500 should be enough for that, right? Then I'll cash in on my awesome idea...
Maybe some projects honestly work out like that. But a lot of the projects I see on these sites fall outside that scope. For big ambitious projects, we all know programmers are not fungible resources, and competent execution matters.
But the worst part is that people humor these projects, or more likely take advantage of the gullibility of the people asking for them... "Yes sir, I can solve that impossible problem promptly and under your budget of $500." It's really sad.