The obvious way to understand quality is to examine what is successful. On the other hand, it is often more helpful to examine what has failed.
Are there any tropes generally considered to make bad science fiction? Some things that pretty much everyone hates? Except for, of course, Mary Sue.
Inconsistency with its own speculative science is another great failing of bad science fiction.
Bad science fiction can have the following characteristics:
It does not hew closely enough to actual science. This is my personal biggest gripe.
It has some ridiculous premise or other (e.g. space aliens come through a worm hole, inseminate all the women of earth, and the offspring are time traveling superheroes). But, as a caveat, some good writers pride themselves on taking ridiculous premises and actually making good stories out of them. Still, hard to imagine a story like that employing good science (#1).
Any quality that makes for bad writing (poor character development, etc) will make for bad science fiction writing
In order to make this question answerable and on topic, let's assume that discussion of anything that can apply to any other genre is out of scope of the question.
Science-fiction breaks down into two categories: hard and soft. Hard Science-fiction is about real science stretched a bit into the unknown. It follows all known rules and then adds something new to experiment with. Arthur C Clarke is a great example of hard science-fiction. He goes as far as inventing new real concepts in his books.
Soft science-fiction is fantasy with a think paint of space, or time travel. All technology could be swapped out for magic and it would not make a difference. The Dune series is a great example.
So bad science-fiction is one that does not know where it lives. One moment it's hard, the next for plot reasons or ignorance of the writer it becomes soft.
Pure soft science-fiction has another pitfall. It often fails to appeal to science-fiction readers if it only becomes a meaningless backdrop for another genre like romance.
