This was quite a hard poll to write, in order to get across the sense in which the words are used, but hopefully it's clearish. In each case, imagine the sentence being said by someone who does not fall into the group the word in question refers to (i.e. the first sentence is not being said by someone gay, etc.).
Which of these do you find offensive?
"I don't like those shoes, they're gay."
"I'm such a spazz."
"He's a complete mentalist."
"What a retard."
"She never buys her round; she's such a jew."
"I can't believe you paid £5 for that - you were gypped."
"OMG, that's so lame!"
"There were these chavs on the train..."
"The score was 7-0 - our team was raped."
"You're such a bitch!"
I was going to put in a question about other words that you find offensive that I hadn't mentioned (other than the glaringly obvious: nigger, paki, fag etc.; the ones in the poll are all things that seem to be considered borderline-acceptable in a way that other words are not) but I wanted to make the first part of the poll private to encourage people to answer, whereas I'd like people to be able to see the other words being suggested, so please do leave them in comments.
My take: I dislike the use of 'gay' to mean 'uncool' or 'crap'; I wouldn't use it myself (and indeed haven't since I was in my early teens with a couple of friends over, who ended up having a conversation with 'gay' this and 'gay' that in front of two of my parents' gay friends, while I slowly died inside), but I probably wouldn't call someone on their use of it. I would never use 'spazz' or 'retard' (though I'm ashamed to say that I used to use both words) and I find them pretty offensive, and I probably would call someone else on it, if they were someone I knew well. "Mentalist", and similar things like "nutter", "loon" etc. I do use, though when I really think about it I'm uneasy. "Jew" in that context is utterly unacceptable to me, and I'd call someone on their usage of it; similarly "gypped", though slightly less so because I do think that some people genuinely don't know where it comes from (I didn't, until it was pointed out to me). "Lame" I don't find offensive, but it's something I'd probably aviod using among people whom I don't know. I hate the word "chav" and never use it, and would probably open up a discussion with someone else used it, though it'd be more along the lines of "wow, I really hate that word" than "that word is offensive" (and I'd be really interested to hear from Scots about the possible offensiveness of the word "ned", which in my head is an equivalent, but I certainly don't understand all the layers of meanings and cultural assumptions. "Raped" in that context is something that it'd never occur to me to use, and I probably wouldn't comment on someone else using it, though they'd likely get a wide-eyed look as I'd be so taken aback. "Bitch" I'm totally fine with, but I know one or two men who dislike it and consciously avoid it.
I have very, very little time for the "but I didn't meeeeeeeaaaaan it like that!" line of defence, and equally little for the "but some of my best friends are [insert group here] and they use that word!" (It's a different matter if someone falls into the group in question, though I would probably still be a bit taken aback by it.) As far as I'm concerned, the only graceful response to being told "hey, I find this word offensive" is "sorry, I'll be more careful with my language in future". It's a basic matter of respect. (Again, it's a different matter if you're in a very close group of friends where you know for absolute certain that no one's offended; I have a couple of fairly off-colour jokes that I will tell around certain people, but would never even consider telling around people I didn't know very well indeed.)