Consequentialism is a theory of moral development which maintains that consequences to actions form the basis of the right and wrong of those actions. In other words, the outcome of an action determines the morality of the action. Consequentialists' problems revolve around the need to define what is good and bad before an action can be assessed. Otherwise, consequentialism commits the fallacy of begging the question.
Also, the problem here is that who decides if the results are good or bad? This is essentially the logical fallacy of begging the question. This error says that you assume that thing to be true that you're trying to prove. So, consequentialism looks at the final result as being good and then judges goodness based on the final result.
Furthermore, consequentialism is far too subjective. What is good for one person, might not be good for another. Therefore, this could lead to immoral acts being considered moral.
Finally, another problem is how many consequences are being observed in regard to an action? One consequence? Two consequences, 30 consequences? At what point does the burden shift from being good to bad or vice versa as individuals look at the ramifications of consequences?
Essentially, consequentialism is a faulty methodology for determining moral right and wrong.
Contrast with deontology.