To illustrate the difference between a timeless short story and a novel, I would suggest you consider Isaac Asimov’s short story ‘Nightfall’. Briefly: in a world of 5 suns, darkness is never experienced. What happens when all the suns are eclipsed? The story is a facinating exploration of how we deal with the unknown. It is clear, concise, and very memorable.

So memorable that John Silverburg jointly with Asimov, expanded the short story into a novel of the same title. In the novel, they authors added expository material and really gave you a feel for the society, gave you a greater sense of the charaters. They added richness with lead up to the cataclysimic event, and after the cataclysmic event explored the reactions to a greater depth.

Is the story the same? Yes.
The novel is far righter, having greater detail and clarity. Both are memorable and both are enjoyable.

This need not be taxing however. Try to read a story like you listen to a piece of music. Perhaps you pick up the bass line or rhythm section at first, and then move to the trumpet, or piano, or vocal section or what have you. I find this technique helps me to read difficult stories without a sense of tediousness, and similarly to read seemingly simple stories with a great degree of depth.

Let’s face it, with so much to read, unless we really put ourselves to it, we won’t reread a short story as many times as it needs and deserves.

There are varying definitions. The 5000 word limit is a very common limit for short story contests and publications. I have seen novellas defined as being as short at 7,500 words. It can be very difficult to publish a short story of over 5000 words because it simply takes up too much room in a print publication.

Here is a web definition that sets the length of a short story at 5000 words:

“a piece of fictional writing usually less than 5000 words that contains these basic elements: characters, setting, plot, conflict, resolution, climax, dialogue, protagonist, and antagonist”
http://www.iclasses.org/assets/literature/literary_glossary.cfm

May I ask where you arrived at the 5,000 word limit? Is that a personal limitation? Personally, I’d consider stories even much longer than that as short stories.

It’s difficult to define them by word count, since most sources won’t give a hard number to their definitions. A Glossary of Literary Terms by Robert Harris is the only text book I know that gives a word count, and even then states it’s a ‘rule of thumb’ that a short story be 20,000 words or fewer. That correlates closely with The Nebula Awards (for science-fiction stories, bringing it back around to earlier discussion), which define the ‘novella’ as 17,500 to 40,000 words (so a short story would be fewer than 17,500).