To me, Farscape is plainly science fiction, not because it has aliens instead of monsters, but because of the way it handles such elements. Point one is that they are handled with internal consistency (compare Buffy's changing treatment of demons -- D'Argo is an alien and not a monster because he's a character, essentially, and Buffy's demons don't really start being characters until quite late on). Point two is that Farscape's world is one that is susceptible to rational enquiry, as Crichton demonstrates nummerous times (in his inimitable fashion) -- the wormholes may be a form of magic, but they are a form of magic that can be investigated and understood and mass-produced. The equivalent in Buffy would be mass-producing Orbs of Thessula. (Arguably, The Initiative were science fiction characters wandering into a fantasy show and finding that their narrative protocols didn't work any more... :)

And please, let's not start saying sf has to have something to do with the future ... :p

Star Wars is a good bit closer to fantasy because it doesn't really do the things Farscape does -- you can't rationally investigate and understand The Force, its aliens (with a couple of exceptions) might as well be monsters, and it takes place in an anthropocentric universe. (Another of the things that I would say makes Farscape science fiction rather than fantasy is the sense that humans are not the centre of the story, or at least that the story is rather larger than the human race.)