Science Fiction that Lasts

Here's an interesting tidbit: James Nicoll on his blog pointed out that Fun fact: Andre Norton's first novel is equidistant between us and the publication of Darwin's Origin of Species. On mentioning that, a friend pointed out that the interesting thing is her novels are still readable, in a way that many writers from the 1950s and 60s aren't. There's a nuge number of SF novels that have aged very badly; either the novels are built on a scientific fact that's later proven wrong (Niven's "The Coldest Place") or posits a short-term future that hasn't worked out (Harrison's "Make Room!"), and a few, like Norton, Schmitz and the like that survive. So what are some of the characteristics of novels that last the ages?

Here's a couple thoughts:

1. Don't sweat the science: sad to say for a genre with "science" in its name, but if you build your novel around some neat new cutting edge scientific theory or technology, you have a really good chance of it being completely wrong. Norton's science is at the level of "and then they pushed a button; likewise is the space opera science of authors such as Schmitz and Farmer.

2. Make your background concepts universal, not contemporary: Norton deals a lot with backgrounds and character problems that have resonated throughout history: war refugees making a place for themselves, the fall of empires, exploration of new lands. Those writers who wrote about the Soviet-U.S. war continuing into the 21st century, or the massive overpopulation of the 1990s, turned out to not be such universal concepts.

I'm kind of developing a couple other rules, but this is what I have so far. Any thoughts?