Utilitarianism is based off of the Greatest Happiness Principle which states that actions are considered moral when they promote utility and immoral when they promote the reverse. Utility itself is defined by Mill as happiness with the absence of pain. The main elements of this philosophy are one's actions and their resulting utility. A person is considered moral when their actions tend to promote utility of the general public in accordance with the Greatest Happiness Principle. However, just an action increasing utility does not necessarily imply a moral action. In order for the action to be moral it must be the optimal choice in increasing utility and minimizing pain. Since it is difficult to determine the superior of two vastly different results, Mill provides us with a system to determine which choice would have the higher quality. This system has the proper judges of the actions determine which they prefer. Whichever is preferred by a majority is considered the action with a higher quality result and thus would be more moral to perform than the action with a lower quality result. In the result of a tie, both choices are considered equally moral. The judges for the actions are those who have had sufficient experience to be able to give a preference. They must have experienced the utility of outcomes to a certain extent in order to properly weigh them.
There are many issues with Utilitarianism, mainly on the Greatest Happiness principle, including the inability to see the future and perfectly determine the consequences of any action, the allowing of malignant practices for the sake of increasing overall utility, the allowing of defying of societal laws for the sake of increasing overall utility, and its disregard of motive. The last one we can discard for now as it would turn this into a Deontology post instead. An example taken from our readings is that of foot binding or bodily mutilation for the sake of beauty. Depending on the views of society, this can be considered moral. Imagine if the world's population were split 50-50, but with one side have 1 or 2 more people. If everyone on the side with the extra people preferred mutilation for the sake of beauty (assuming that everyone was capable of proper judgment), then no matter what the other people thought, Utilitarianism would allow it. This is also true even if there was a law placed by society as society's happiness is more important than its justice for Utilitarians. As for the indeterminate future, when considering foot binding, it is generally done by mothers to ensure their daughters attain a proper husband. The assumed utility produced would be caused by said attainment, but in the case that she is incapable of doing so, the mother's choice has become immoral as the utility desired was never attained and excruciating pain was inflicted upon the daughter.
The Greatest Happiness principle in general is good, but it has many flaws as any ethical systems does. Due to our inability to perfectly predict the future according to our actions (assuming he future is capable of being altered with our actions), the results we desire are capable of, and often do, fall short of what was intended. If unforeseen parameters caused all of our actions to backfire, even though we were attempting to act in accordance with Utilitarianism, we would all be considered immoral as our results only caused pain. If this happened to everyone in the entire world, then no man could be considered moral. The Greatest Happiness principle also allows for us to cause pain to others as long as a majority of the people become happier. We could essentially just steal resources from smaller foreign countries and drive them to poverty as long as more people benefit than lose. Things such as slavery, bullying, rape, racism, and murder could be justified under Utilitarianism as long as the majority prefers it. Murderers could justify their action by simply killing all of those who opposed them. Once their numbers became the majority, murdering became justifiable as moral. Lastly, the Greatest Happiness principle eliminates the usage of the laws provided by our government. As long as the person's actions increase general utility, then it does not matter how many laws are broken in the process. We could all go speeding down roads and ignoring traffic signals/signs to our full enjoyment despite there being speed limits as long as few people cared and most people would be having a blast.
All of these examples display cases where the Greatest Happiness principle would "fail". It fails in the sense that the standard of what is/isn't moral can be easily changed in society's eyes, and as long as the results produced are in accordance to what the majority prefer, then all preset laws and individual preferences would be considered invalid and can legitimize practices that we currently see as immoral.
Utilitarianism is based off the idea of utility. Mill defines utility as pleasure and the absence of pain. Furthermore, Mill states that Utilitarianism follows the Greatest Happiness Principle where actions are considered moral when they tend to promote happiness and deter its opposite, and immoral when the opposite occurs. The ultimate goal is to maximize the overall utility of the world or universe. Utilitarianism's main focus of judgment is the result of our actions, and not so much on the motives behind them (although acting in accordance with Utilitarianism without succeeding can provide its own form of happiness). These actions are judged mainly based on the quality of the utility that it would provide, which is based off the preferences of the general populace, at least of those that are affected. If the majority prefers one result over another, then it would be considered of higher quality, and our choices should always be aimed at the action that produces higher quality utility.
The main issue I find with Utilitarianism is the morality of those who act against it, but end up with a result that is in accordance with it. According to Mill, if the result is the same, then the morality of the actions are the same no matter the motive behind it. This also means that a man who acts with motives of equal strength in "goodness" as another, but produces a result of much lower quality utility, then the man would be considered much less moral.