Truth is stranger than fiction, and more exciting, too.
I think you are underestimating the value of 'buzz-and-zap' as a contributing factor to greatness. If our "imagined literate reader" is too parochial to appreciate the vertiginous, spirit-crushing time-spans that Aldiss provides in his
great Helliconia series, then the lack is in him or her.
Worldbuilding and 'sensawunda' and good old speculation are as precious and important as anything you might find in the kitchen sink
"Vertiginous, spirit-crushing time-spans" do not in themselves make greatness in a work, and the proof is the number of other books, many rather awful, that have such an ingredient. It is how a given author works with his or her ideas that makes greatness: a master can make a moving and memorable tale from the most ordinary of events (as, say, with Joyce's
Ulysses), while a hack cannot be rescued by any number of wild concepts (as witness the piles of slop from the pulps--not all pulp material was slop, I'm not saying that, but a very great deal was).
Nor, I would say, do worldbuilding or speculation
inherently make for greatness in fiction. Worldbuilding can be literally wonderful, as with a Jack Vance--or it can be mind-numbingly boring, however carefully worked out and "correct". As to "speculation", and, for that matter, "sensawunder", I at least find sheerly factual non-fiction works--those of folk like
Brian Greene or
Lee Smolin or
Michio Kaku--to be far more appealing, chiefly because they speculate on what may well be
exactly true, which is drastically more exciting than invented things. The value of invented things, of whatever an sf writer uses that makes the s for the f, is the extent to which they allow that writer to better shine light into whatever aspect of the human condition that he or she wants to explore with us.
In short, greatness lies in the author, not the subject or its setting. If a great author writes sf, it will (usually) be great sf; if a mediocre author writes sf, it will be mediocre sf. Conversely, a truly great sf author could (almost certainly) write great non-sf fiction, while a mediocre sf author is not likely to ever write any but mediocre non-sf fiction.
We can disagree about whether Brian Aldiss is great, but whichever it is--great or not-great--is not owing to his subject matter.