"it's retarded" appears to take on the place in a paragraph where an actual argument or reason would go, but it isn't. It's just a childish phrase used to indicate dislike for something. If you don't like the "it's offensive to use a phrase indicating some form of developmental disorder as a pejorative", then please consider the argument that it accomplishes and communicates nothing except the fact that the speaker / writer lacks the ability or motivation to articulate his objections.
However, if you're attempting to get a point across, you generally do better by keeping social norms in mind. Offending your audience instantly makes them think about whatever "horrible" thing you just said and not on whatever point you're trying to communicate.
Put another way, if nobody is getting your point, perhaps the problem isn't with the audience :)
(I am, however, going to steal that "offense is taken, not given" line for future (ab)use)
> However, if you're attempting to get a point across, you generally do better by keeping social norms in mind.
And if everyone followed that principle, society would never change. The perfect fourth would still sound dissonant, rape victims would be under even more pressure to keep the crime secret, etc.