Here we go again. I've complained before about the undeserving admiration given to many science fiction novels and short stories of earlier decades. Such reverence frequently results in fans recommending these works to people wanting to try the genre. And that's not a good thing. Readers new to the genre are not served well by recommendations to read Isaac Asimov, EE 'Doc' Smith, Robert Heinlein, or the like. Such fiction is no longer relevant, is often written with sensibilities offensive to modern readers, usually has painfully bad prose, and is mostly hard to find because it's out of print. A better recommendation would be a current author - such as Richard Morgan, Alastair Reynolds, Iain M Banks, Ken MacLeod, Stephen Baxter, and so on.
I can hear howls of outrage across the tinterweb.
And so I say again: holding up Foundation or Second Stage Lensman as good introductions to sf will no longer wash. They're historical documents. In those days, science fiction was a different place; they did things differently. And many "classics" of those days do not fare well when compared to modern works.
I recently reread 'Nightfall' by Isaac Asimov, in the anthology A Science Fiction Omnibus. 'Nightfall' was first published in the September 1941 issue of Astounding Stories. In the story, the world of Lagash has six suns, and only ever experiences darkness once every 2,049 years. A group of astronomers have calculated that a "night" is imminent, and realised it's the cause of their cyclical history.
I vaguely recall first reading the story when I was around eleven or twelve. I've long been aware of its status as a "classic", of its reputation as one of Asimov's best stories. So I was surprised on my recent reread to discover that it's, well, it's pretty bad. Asimov's prose was clunky at best, and it's not his best in 'Nightfall'. The world-building is lacklustre and slipshod - characters have names like Sheerin 501 and Beenay 25, and that's it. In all other respects, it could be set in 1940s USA. The ending - the darkness and resulting panic - is given away on the first page. Much of the "idea" is explained in conversation by the cast. The narration even pulls out of the story at one point, destroying the compact with the reader (ignore the bad grammar, a sentence fragment wodged onto a sentence with a semi-colon):
"Not Earth's feeble thirty-six hundred Stars visible to the eye; Lagash was in the center of a giant cluster."
By all criteria, 'Nightfall' fails as a good short story. And yet it's still regarded as a classic. Some people will even suggest it's a good example of science fiction. Rubbish. It's built around a single, not very interesting idea - a world has never seen darkness... and then it gets dark. Wow. There's a comment on the boom-bust nature of civilisations in there, but it's pretty much thrown away. Asimov uses it in much more detail some ten years later in Foundation, anyway.
In part, this harkens back to my earlier post about the primacy of idea in science fiction. 'Nightfall' contains a very obvious idea and it appears to me that many think the sheer in-your-face nature of it overrides all the story's faults. Which should not be the case. A story should be considered a classic for a number of reasons - continuing relevance, good writing, originality (in ideas and/or deployment), rigour (of world-building, of story), meaning, impact upon the genre, impact upon the reader...
Shining the spotlight upon idea leaves all else in darkness (seems an appropriate metaphor for a piece citing 'Nightfall'). In fact, the more an idea or trope is used, the more polished it becomes, and so the higher its albedo.
The howling is becoming deafening now, so I'll finish by saying I don't think we should refuse to read old classic works, but we must recognise that they're historical documents. And add that caveat to any such recommendations or commentary. Further, modern sf readers shouldn't need to be aware of everything which has gone before, but modern sf writers certainly ought to.