I enjoy science fiction and fantasy. Personally, I don't see a real difference between the two -- science fiction just has a fake "science" explanation for the magic.
Now, some science fiction is more science-y than others. For instance, Firefly is very, very careful about not throwing around unrealistic things -- there are no explosion noises in space, the ships move with real zero-g physics, and the merchant ships have no laser guns because why would they? (But I don't really believe that Reavers would ever be a thing.) Meanwhile Doctor Who just throws around some techno-jargon about a "chameleon circuit" or a "sonic screwdriver" and we're just supposed to believe it. When the Doctor needs to do something, it suddenly becomes scientifically possible. John says that makes it fantasy. I said, "No it's not, he just said the word 'DNA' so it's science fiction!"
Really, that's the only difference in my mind. Because almost every science fiction story ever (Firefly being a notable exception) includes faster-than-light travel, even though it's well-known to be impossible to go faster than light. I actually subdivide science fiction in my mind based on how they go faster than light -- is it with some magical warp drive, turn it on and the rules no longer apply to you? Or is it some kind of wormhole idea -- more theoretically possible? Lois McMaster Bujold has wormholes in her fiction, which results in a lot of fun possibilities (what would happen to a colony established by Earth if the wormhole that got you there spontaneously collapses?), but I just can't figure out how the people in her universe ever discovered them. You see, to go through a wormhole, you need a ship equipped with Necklin rods (which have no purpose other than to take you through a wormhole) plus a pilot with a cybernetic implant which allows him to navigate the five-dimensional space in there. Cool, no problem. But which came first, discovering the wormholes, or inventing the Necklin rods? Without the Necklin rods, how would you know the wormhole was there? But why invent Necklin rods with no wormholes to go through? And even if you had those two things, what happens when you stumble upon the wormhole, turn on your Necklin rods, and then get lost in five-dimensional space because the pilot doesn't have implants?
And then there's the issue of where the wormholes are -- in orbit in the system, or stationary? Because it's odd to say "the planet is five days' trip from the wormhole" when the planet, of course, is in motion. Unless the wormhole is in synchronous orbit along with the Earth, or orbiting around the Earth, the amount of time it takes to get from one to another will be highly variable. On the way to visit your grandma, it might take a week; on the way back, six months.
Timothy Zahn has a cool solution in his Quadrail series -- there is actually a train that goes faster than light through some kind of trans-dimensional tunnel. You spend days traveling to the edge of your system to get to the station, but once you hop on the train you could be in the next system over in an hour or two. But how they make this space-tunnel work is beyond me.
Another glitch is how easy it always is to get from orbit to the planet's surface and back. People in sci-fi always seem to think nothing of it, but it takes a massive amount of fuel to boost your mass out of a planet's gravity. For economic reasons, I can't imagine people would drop back down to the surface just to grab a screwdriver, but that's what they always seem to do! (Arthur C. Clarke's Fountains of Paradise solves this problem with an elevator leading to an orbital space station. If you could make a strong enough cable, this actually would work.)
A transporter, though, is not much of a solution at all. Turning matter into energy and back again is a big deal. The replicators in Star Trek are even worse. Want a cup of coffee? No problem -- the machine will turn energy into coffee-shaped matter for you in seconds! But ... do you know how much energy that would take? The exchange rate between matter and energy is literally astronomical -- it's the speed of light (a really big number) squared. (Thanks, Einstein!) So unless you have an actual sun blazing away in your engine room (and perhaps even if you do) it just is not practical to make all your food and dishes and clothes this way.
Then again, if you then use everyone's dirty clothes and dishes and poops in a fusion reactor, could you compensate for it that way?
Hmmm. The idea has merit, if you don't mind your Earl Grey Tea, Hot being your own reconstituted urine. But it still seems in the long run it would be more practical to wash your dishes, launder your uniforms, and use hydroponics to grow food. Why would you transfer matter to energy and back again, except just to show off?
Many of the other things in science fiction are theoretically possible. For instance, genetic engineering is in its infancy, so there's no way to be sure that we can't engineer pretty much anything we want. On the other hand, it's quite possible that in 50 or 100 years, we'll laugh our heads off about "genetically engineered super-soldiers" the way we now do about "radioactive spiders." That is, once we know more about the science behind it, we'll know that sort of thing isn't really possible. Hopefully. It would be pretty freaky if it were.
And then there's the "but why would you do that?" technology. In the future, everything, everything is 3D. Is it really convenient to have 3D pictures on your wall? It seems 2D remains, not because 3D is hard to produce, but because it's just simpler a lot of the time. Writing is 2D. There is no advantage to a 3D computer console if you just want to read your space-mail.
Ah, but of course no one will read in the future -- everything will be audio, video, and 3D! Nah, I don't think so. We have technology now to turn the written word into audio for you, but no one uses it but blind people. It's just so much slower and harder to process a lot of information when you have to fastforward instead of just scanning over the page to the part you want. No way are we going to retire books and have shelves full of vid disks -- even if that weren't a really stupid way to store your digital files. I refuse to believe in a culture that developed faster-than-light travel but can't store a video more compactly than a disk you can hold in your hand.
Then of course, consider this: how long does it take to write a chapter of a book on, say, physics? And how long does it take to make an hourlong video giving the same information? Sure, you could do a video of just a guy talking, but if you want diagrams, different actors, and so forth, the manpower needed is a bit much. You've have to have people clearly preferring to learn by video over text for it to be economical -- and the reality is, a lot of people just prefer text. I do. Either way, I assume text, audio, and video will continue to exist side-by-side for years to come.
So, maybe stop dissing Doctor Who for having a TARDIS that's bigger on the inside and travels through time using mysterious energy. All science fiction is just fantasy with technobabble added. We suspend our disbelief to enjoy it -- that's sort of the point.