Just recently I found myself listening to a reading of [*Nightfall* by Isaac Asimov](http://escapepod.org/2007/04/05/ep100-nightfall/). This is a fabulous story that I’ve read many, many times. But the slower pace and forced attention of the audio book led me to notice a change in the tone compared to the SF stories that I normally listen to.
It’s a little bit hard to pin down exactly what it is that’s different. Part of it is probably the product of the story being written around 66 years ago. But there are some other parts that point towards the evolution that the SF form has seen.
Short fiction in SF is viciously innovative. The pure short story category, those stories under 7,500 words, sees some of the most interesting experiments in viewpoint, approach or theme. But on the flip side of the coin, those stories that simply try to take an existing idea and develop it more completely are not often very popular.
So this gives modern short SF a different feel to that of the WW2-era. For instance, you could never present an alien in short SF today without making some attempt to provide some element of culture or language to underline their alien nature. But in the era of early Asimov or Heinlein it was possible to just tell the story without worrying about ticking all the innovation boxes.
(In part, of course, because the relative youth of the SF genre then meant that almost all new stories would be innovative almost by default)
I don’t think this change is a bad one, and it certainly doesn’t diminish the classic work of earlier generations of writers. But it does mean that you can’t easily leap from one to the other in reading or listening, as you can suffer the intellectual equivalent of a pulled muscle as the story changes gear.