TBH, I don't usually find any subject to be off-limits, but occasionally something won't appeal to me. Whether or not it was funny can make a big difference.
For instance, a racist, or homophobic joke is cool in the right context, and if it's funny. In the wrong context, and if it's not funny, I can find it boorish.
I have told more than my fair share of jokes that I would consider boorish, so I'm in no position to take the high ground.
A silly example off the top of my head, a story that made a bit of news recently was the story of a footballer who made a joke on twitter about the gay rugby player Gareth Thomas.
He said something along the lines of "I wouldn't want to be in the bed next to him."
I didn't think that was funny at all, and it appeared pathetic and boorish to me. Not because it was a joke about gays, but because it was such a feeble attempt at humour, and just came accross as homophobic.
If the joke had been clever and funny, I would have found it funny.