Morality As a Limit
Hedonism is an ethical system. It holds pleasure as the standard of value. There are many variants of hedonism, with different interpretations of what "pleasure" consists of. The two basic categories of pleasure are physical pleasure, and psychological pleasure.
The physical pleasure variant of hedonism commits a grave error. It denies the importance of man's means of survival, his mind. It requires the use of one's mind in the pursuit of physical pleasure, but it subverts it by enslaving it. Very few people have tried to support a purely physical hedonism. It rejects the need for happiness.
The psychological pleasure variant of hedonism is flawed as well. It leaves unspoken the source of psychological pleasure. By treating it as a self-evident primary, it abdicates its status as an explicit ethical code. Since the sources of psychological pleasure end up as the actual guide to action, hedonism amounts to a rationale for doing what you feel like. It is not a moral code because it doesn't identify which values to pursue. It accepts whatever value-judgments you've already made, and treats them as axiomatic.
Psychological pleasure stems from the achievement of the emotional component of values. When you feel that something is valuable, and you achieve it, it gives you pleasure. A rational moral code aims at making sure the emotions you feel are valid. That you value that which is really in your interest. Hedonism does not allow an evaluation of your values, no matter how destructive or life-threatening they may be.