I think of taboos generally as zones of discomfort. There are lots of them, and they vary depending on the culture. I mentioned them in my earlier post on humor, because a lot of humor centers around taboo borderlines of various kinds, including (but certainly not restricted to) body parts, bodily functions, illness and death, race, religion - the list goes on and on. I think the movie that was the most bipolar ever for me was Monty Python's The Meaning of Life, exactly because it took aim at so many body taboo borderlines. Thus, when I liked the jokes, I loved them - and when I didn't love them, I was appalled by them (stomach upset has never been something funny for me!).
Monty Python's parrot sketch played with death, but managed to avoid any of the gross-out aspects of the topic and instead played with euphemisms. I love euphemisms, so I guess it's no surprise that the line "If he hadn't been nailed to the perch he would be pushing up the daisies!" had me incapacitated with giggles. People think of lots of ways to talk around taboo subjects, and as time passes, they continue to have to find more, because the association of any given euphemism with a taboo topic effectively contaminates it over time. Think of the number of different names that have been given to toilets and the rooms in which they reside. Or think of the number of different names that have been given to former slave and immigrant peoples of the USA, which have then been tossed out and replaced with others as they were judged too derogatory.
When I was living in Japan with my husband a movie came out that portrayed the final days of someone with a terminal condition. The odd thing about this movie for us was that it was a comedy, and one of the main jokes was the fact that the guy was dying and his family couldn't tell him. I'll admit I was a little shocked by this. However, when we asked our friends we learned that in Japan, doctors will not tell patients that they are dying because it would be too upsetting; instead they tell the family so that they will be able to prepare. This is something that makes my gut go "no!" but from an anthropological point of view I can see how it would make sense culturally.
Taboos can result in a lot of cultural self-restriction. The area of technology leaps to mind, where the ethics of cloning and stem cell research play such a large role in determining where money goes, and thus where the technology goes next. Religious restrictions can have a deep influence in many areas of a culture - in Islamic art, for example, where the depiction of living beings (particularly humans) has been historically discouraged as blasphemous.
Taboo is rich territory for world and culture building. The planet of Garini in my story "Let the Word Take Me" had a religious taboo on the use of language. Many religions restrict the utterance of particular phrases or names, but in the Garini case the taboo applied to any free use of the language. Check out the July/August 2008 Analog magazine if you want to see how it worked; I don't want to give too many spoilers here. Frank Herbert (Dune) does a brilliant job of culture building with Arrakis and the treatment of water there, building in taboos on wastage that are treated with respect or disrespect by different power groups. Ursula LeGuin does some fascinating work with taboo in The Left Hand of Darkness when she creates a race of ambi-gendered humans (no time here to explain the exact details; go read it if you haven't) and builds folktales for them which elaborate on the kinds of taboos they might have, including for example incest and childbearing.
I encourage writers to think about the taboos of the cultures they create, because this can be a great way to give a culture extra dimension, to link its social groups in principled ways and to make it feel grounded in a physical environment.