Why should 5 countries have anymore power than the other 189? Read about UN resolution 67/19 (a vote on whether on not Palestine should become a full member of the UN). The first step, a vote on whether it should become an observer state, was passed 138-9 at the general council (the US obviously voting against). To become a full member, however, Palestine had to be approved by the security council. The US vetoed the resolution, even though there was an overwhelming approval. The veto power is a brake to democracy.
The veto power of the permanent members holds other states hostage because they can consistently use their veto powers to back votes that are in their own best interests. A good example is the US supporting Israel in every action in the UN in the last 50 years. The veto defeats the purpose of having a democratic vote because even if other nations vote 100 to 1 they can still be overturned by veto.
The six member nations of the United Nations security counsel should be entitled to retain their veto power because those countries are the ones called upon to provide the most troops and money toward any military effort. Allowing them a veto provides the participants with the most to lose from intervention the ability to refrain from such action after careful consideration.