I think the answer choices used can greatly affect the difficulty. For example, the infamous Pokemon question (still my favorite question in all of Millionaire). The question asked which of the following was not a Pokemon, with the correct answer being Frodo. However, this question appeared before Lord of the Rings was released as a film, so the Hobbit connection was not as salient. If asked today with the same choices, I could see it as 16k, maybe 32k. However, if the fourth choice was from Digimon or Yu Gi Oh (anachronisms aside), that question probably could have been bumped from 500k to 1M.
One of the best 1M questions would have to be the Carol Brady maiden name question. The choices included Sam the butcher's last name, Alice's last name, Martin (the last name of Carol's first husband), and the correct answer (which I don't remember). I was certain it was Martin, completely forgetting that that was her first married name. My mom and my sister, who were watching with me, were able to trace the answer as being from the grandparent's episode, but didn't know which answer was right (the above connections were after the fact, BTW).
So to answer your original question, the 1M question should contain a subject that most people would be familiar with (computer bugs, the song God Bless America, walkmans) but not necessarily well read on the subject. If a person is likely to be well read on the subject (The Brady Bunch for example), the choices should throw a wrench into your thinking, possibly to confuse you.