I have been working over the past decade or so on an entirely unscientific set of general principles that famous quotations tend to follow. Why, I don't know. Maybe it's because people tend to like repeating these quotations, buying books full of them, using them as prooftexts for various arguments, etc. Which is not to imply that I am not one of these people. So here I will share with the world the following three tendencies:
1. Many famous quotations have multiple attributions.
I discussed an instance of this in a previous post. I think this breaks down into two subclasses: a) quotations that have are clever in a way that multiple people can come up with it independently, or which have perhaps been heard and repeated, and the repeater's utterance becomes famous. This is as opposed to b) quotations which, by virtue of being clever and witty, get mistakenly attributed to someone else known for saying clever and witty things. I'm suspicious whenever I read a quotation attributed to Woody Allen, Groucho Marx, Winston Churchill, Oscar Wilde, Mark Twain, or Abraham Lincoln. It seems that these people in particular are virtually interchangeable: which one of them said, "I don't want to be in any club that would have me as a member"? If you're me, and I know I am, you've seen it attributed to all of them. For the record, Groucho did indeed claim in Groucho and Me to have sent a telegram saying this, which doesn't mean that he came up with the phrase, or that he ever sent such a telegram. Also, Woody Allen has used this line a number of times (perhaps most famously in "Annie Hall"), obviously in tribute to Groucho.
2. Famous quotations often become altered in transmission, usually improving them.
Ever heard anyone say, "I am as mad as hell and I am not going to take it anymore"? Of course not; what people say is "I'm mad as hell" etc., although the actual line (from the movie "Network" -- worth watching only as a cultural artifact) is the former, vastly inferior version. Wikipedia has an impressive list of "misquotations," many of which follow this axiom. What I like about this is that it shows that there is an innate sense for pithiness and elegance leading people to improve on already clever phrases without even knowing they are doing so.
3. Archaic quotations (Shakespeare, King James Bible, etc.) are often obscure.
This is the least scientific of my observations, based solely on the shaky foundation of my subjectivity. But think of the following quotations:
"Suffer the little children to come unto me."
"The lord is my shepherd; I shall not want."
"For now we see through a glass darkly."
"Get thee behind me, Satan."
"Wherefore art thou Romeo?"
"Cry 'Havoc,' and let slip the dogs of war"
"We are such stuff as dreams are made on, and our little life is rounded with a sleep."
I think that these quotations draw a sort of spooky power by not really meaning much to contemporary ears. Maybe that's why some people, including me, instinctively feel that the King James Version of the Bible just feels more biblical than more up-to-date versions. How could it be the Bible if it makes sense?
So there they are. I feel like there are better examples out there, but I can't think of them offhand. Maybe I'll come back to this later.