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.