I don't like "sad" in it's newest meaning ("to be regarded with contempt") - it's the attitude involved that I find distasteful, which is covered by Peter T's third category ("does the new word capture something ugly in the society that we don't like, and we attack the word because it refers to the thing we don't like?").

It also comes under a fourth catergory which he didn't mention, which is, "does this use of the word take away our freedom to use this word in an existing and valuable sense"

That's the reason why so many people regret the reassignment of "gay" - it's not that there weren't valid reasons for people wanting a new non-pejorative word for homosexual - but "gay" was a really handy word where it was (and especially good for rhyming purposes), and there isn't really a replacement for it that I can think of. "Jolly"? "Lighthearted"? Neither means the same - or sounds it either. Too ponderous by half.

Within a few years I suspect "gay" will have worn out it's newer meaning, and be dropped - but I doubt if it will ever come back to its earlier meaning.

So that will mean we'll have lost both sad and gay.