What is the difference between different words that mean the exact same thing, in that some are considered obscene and others are not? I would suggest that there is nothing inherent about the meaning or connotations of these words that differs from other synonyms that are considered more acceptable, but that they have merely had the bad fortune to become commonly used for offensive or shock effect. This has given them an unfairly sullied image.
I agree with you, Izzy. In fact, the concept of a “bad word” is nonsensical to me. The word isn’t bad, it’s the thought that might be behind it. Saying “F***!” as an exclamation of frustration should be no worse than “Darn!” OTOH, saying “Screw you!” should be just as bad as “F*** you!” because it’s clearly a personal attack. I can say “crap” and “poop”, but not “sh**”? It’s just silly.
All languages contain obscenities, but there’s nothing inherently obscene about a given obscenity; it’s just sociolinguistic weight. The word “shit” wasn’t even particularly obscene until the Victorian era.
Conversely, in Mexican Spanish, due to its use in the extremely severe insult “¡Chinga tu madre!”, the word “madre” (mother) is as obscene as the word “fuck” (or perhaps a better parallel would be “bitch”, which is not obscene in certain limited contexts but in general is). If you want to refer to someone’s mother in a polite way, you have to say “mamá”.
So if mothers can be obscene, there’s nothing inherently obscene about actual sex that makes the word “fuck” obscene. I predict that as more and more people use the word “fuck” openly, it will start to lose its edge, possibly becoming no more obscene than “damn” or the like.
(Its replacement? “Felch.”)
I think that while the examples above hold true, there is a recognized element of intent behind someone’s choice of words. There is a connotative difference in some cases. If I say a certain person is a poopy-head, he will not take nearly asa much offense as if I call him Shit-for-Brains.
If I were 6 years old, and you called me a poopy-head, I’d be pissed! 
The vulgarity of a word/phrase may be from the common, sub-standard, low-class people that used them. I’m speaking historically here…when a class structure was much more prevalent. Maybe the upper-class would gasp at the use of such words because they were plebeian. Hence, the dual definition of “vulgar”.
I notice that a bunch of movies edited for TV nowadays take extremely ridiculous steps in censoring swaer words. For instance, if a character says “Asshole!”, they will often just cut out the “hole”. WTF?!!?!?
SeatTime says:
That pretty much nails it. The obscene words we have today tend to have roots in older versions of English and were replaced with French or Latin words that were more accpetable. E.g. shit vs. feces, fuck vs. copulate, etc. My guess is that it happened about the same time as the French-speaking Normans invaded the British isles. The Normans, upperclass by conquest, spoke French. The defeated Anglo-Saxons became lower class, so their words became “vulgar”.