Speaking of things that are funny to some and not others, an instructive example is the Orange Head joke. Usually when it's told, the audience is sharply divided into those who think it's hilarious and those who struggle to see what's funny.
Here's the Orange Head joke:
It's business as usual for a bartender, and one day as he is cleaning his bar when an unusual customer walks in. The man is dressed in an expensive suit, has a beautiful supermodel hanging off each arm, and has a limo parked outside. Furthermore, the man has an orange for a head.
The customer sits down at the bar and orders everyone a drink. He pays for it from a roll of hundreds and manages to get the attention of every woman in the joint, despite having an orange for a head.
The bartender is not a man to pry, but he feels compelled to ask about this man's life.
"Excuse me," says the bartender, "I can't help but notice that you're obviously fabulously wealthy and irresistable to women, but you have an orange for a head. How did that happen?"
So the man told his story.
"A while back, when I was penniless, I was walking along the beach and saw an old lamp, half buried in the sand. I picked it up and gave it a clean, and POOF! out popped a genie. The genie explained that he had been trapped in that lamp for two hundred years, and that he was so grateful to me for freeing him that he would give me three wishes.
"For my first wish I asked for an unlimited fortune. The genie said 'It is done!' and from then on, whenever I needed money, it was there.
"For my second wish I asked for the attention of all the most beautiful women in the world. The genie said it was done, and since then I have been able to get any woman I wanted.
"For my third wish -- and, this is the bit where I kinda fucked up -- I asked for an orange for a head."
Do you think it's funny?
If you search for this joke's key words, you'll see many pages where, after it's told, people react incredulously and ask where the joke was. Others at the same time are laughing their heads off. Here's a blog post that attempts to analyze this, though it doesn't get far.
(I personally think it's hilarious, and easily the best joke I heard last year. When I retold it at my blog, I got many concurring comments, but also comments from people who didn't see anything funny, even after those who did tried to explain what they found in it. Several people went on to convince themselves it's garbled and there must be an "original" version in which the final remark makes sense and is funny - and offered several ideas of how it might go).