The Donald says "Nobody knows why they're protesting, but they're having a good time." To give him credit, he's half right about the "having a good time" part. As for the "nobody knows why they're protesting part," I'll let Alan Grayson answer that.
That's the short version. Keith Olbermann reads the long version.
To read the list of grievances, surf over to the Coffee Party USA post on the topic, then realize that Occupy Wall Street is a sustainability movement, even if they haven't figured that out yet. If you don't believe me, stay tuned and I'll show you.