Simple. Utilitarianism is concerned with "the good of the whole," "the greatest happiness," and "the ends justifying the means."
If you're too concerned about the good of the whole, you neglect the good of the individual...a mistake, as individuals collectively make up the whole. Not to mention that individual capability is a powerful thing.
As for "the greatest happiness" bit, one's person's happiness may not be the same as another's happiness, seeing as they are different people and have different perspectives, not to mention that trying to quantify something that is innately qualitative is kind of retarded. Trying to achieve someone's warped idea of what the greatest happiness for everybody makes no sense whatsoever. You can't achieve a uniform definition of happiness for everybody. This can be dangerous in that a government could use this as a justification for their actions however heinous or reproachable by saying that their means are only a way for their society to achieve happiness, when in reality that "happiness" is its definition of happiness, not those of their citizens.
As for "the ends justifying the means" part of it, it's pretty self-explanatory...you could use "brave new world" as an example...for the society in that book, it is acceptable to deliberately alcohol poison fetuses in order to lower their mental capabilities so that their society has more lower class workers and is thus able to function properly. The end is "good"--a working society...the means--not so much...but utilitarianism says it's ok...
philosophy lecture class last summer heh what i remember, at least