It is appropriate to criticize Wall Street for its excesses. However, most of the time Wall Street is used (by politicians) as a convenient scapegoat - due to Wall Street's unpopularity, and because public sentiment can reliably be turned against "those greedy fat-cats". Quite often, it masks the actual problem(s).
I like to think of Wall Street as part of the "plumbing" that services a capitalist economy. So, why is it fashionable to blame the plumbing?
Most of Wall Street's activities are based on the transfer of risk from producers and consumers onto people who are prepared to take on speculative risk - in the hope of substantial rewards. Despite the fact that Wall Street doesn't make any tangible products, it provides a valuable service by creating a marketplace that rewards productive activities. And, it is the middle class (as well as the rich) who benefit from investment.
One could argue that the main advantage that Americans have (over the Rest of the World) is the amount of capital that is invested (per capita) in America. Otherwise, how would one explain the difference between a construction worker in America (who has a $250K piece of machinery working alongside him) vs a manual laborer who works on construction projects in the developing world, with no more than 'muscle capital' behind him?
Someone had to make a capital investment of $250K to make the American construction worker more productive, allowing him to earn $25/hr. More often than not, it is a publicly traded company that makes such investments, with funding raised on Wall Street. So, indirectly, all of us owe our standard of living to the fact that we have a marketplace for capital that is more-or-less free.
So, what causes the system to not work properly sometimes?
Politicians - many of whom are beholden to corrupt Wall Street fat-cats - are allowed (and sometimes encouraged) to make stupid laws. Of course, politicians produce neither goods nor services; their blank-check comes from their taxing (or borrowing) authority. Some of these laws 'insure' the fat-cats from risk - effectively transferring the risk from Wall Street to the tax payers [usually without the consent of the tax payers]. Well, they do have consent in the form of votes, usually earned on the basis of false promises and soaring rhetoric.
In addition to legal cover for Wall Street corruption, corrupt/stupid politicians also create entitites such as Fannie Mae, Freddie Mac, FDIC, FSLIC etc., which complete the illusion of safety that corrupt WS fat-cats need to fleece the public.
In the above chain, the proximate cause of the problem is the fact that politicians are given the ability to meddle in areas that should normally be outside their constitutionally prescribed areas of operation. If they were bound by the constitution, they would not have those powers, and they could not dispense favors to Wall Street fat-cats, lobbyists etc. But for the efforts of politicians, the usual 'buyer-beware' type of caution that exists in all marketplaces would keep people from falling prey to the get-rich-quick schemes that corrupt Wall Street fat-cats seek to propagate.
Needless to say, politicians are unlikely to blame themselves - or the fact that they have usurped unconstitutional powers that they dispense as favors - when easy targets such as Wall Street and lobbyists are around for their scapegoating pleasure!