Common refrains I hear from sci-fi fans are both “I want the aliens to be more alien” (often when referring to certain film or TV aliens) and “You have to check this out: the aliens were really alien” (often when referring to certain books). The latter sentiment makes sense, because when you start considering how evolution might have taken place on other words, the bilaterally symmetrical humanoids that dominate much of cinematic science fiction seem less likely. In fact, there might be a whole host of unstated assumptions about anatomy and body chemistry that are very Earth-centric.
These are some of the trains of thought astrobiologists are busily boarding, as detailed in an article by Sarah Scoles for Scientific American.
This is the sort of thought experiment that I’ll always find exciting, because one day, we’ll find out just how right or wrong we are. In the meantime, I might try and find a copy of the entirely fictional, but enormously enjoyable Barlowe’s Guide to Extraterrestrials.