This may sound strange seeing as SciFi is a genre full of the completely odd and strange occurrences but that doesn't mean that logic isn't important. Things can't just happen for no reason, there has to be some logic behind it. Say sky has fallen in your story- why? It can't just fall and no one knows why.

Before writing, make sure everything 'abnormal' in your story has a reason behind it. Mutant aliens, why are they there? Disease that has killed millions, how did it occur?

This may sound simple but a lot of writers leave things out which in turn confuses the writer.