It's a lot of different people with different beliefs. The majority seem upset about some combination of:

1) growing income disparity (http://www.slate.com/articles/news_and_politics/the_great_di...)

2) regulatory capture by financial institutions (http://papers.ssrn.com/sol3/papers.cfm?abstract_id=892925)

3) the use of public funds to subsidize risky investment (http://www.thenation.com/article/153929/aig-bailout-scandal)

4) apparent double standards for the rich (you certainly face more jail time for robbing a bank than for ruining one)

I think that most people can agree that these are significant problems. It's actually amazing how little has changed since the 2007 mortgage crash.