As a writer of sci-fi, I was going to reply to this interesting thread, but Banzai has it so bang on the money, all I'll say is 'see above'.
It all depends on what kind of story you want to write. Color TV was a scientific impossibility fifty years ago, so as long as you have even a bit of logic in your technology, you can write whatever you want. Wormholes are fun if you want the story to happen mostly on planets (Like Stargate) but slower than light travel opens a lot of possibilities, like pirates, commercial routes between planet, pursuits and all that stuff (Recommend watching the TV show Firefly, it was canceled but you should be able to find it if you look) although that reduces your field of action to a single solar system (Still really very big, though, with maybe dozens of planets and hundreds of moons). Aerodynamics are a must if your ship is going to land on planets, but otherwise, it's quite useless. Bear in mind though that your giant flying box will be stuck in space forever as entering atmosphere would most likely be a one way trip. Lack of gravity and its effect on the human body... I think you'll know more than I do, but you gotta remember that large ships have gravity of their own. Lower than Earth's, but still... And I saw somewhere that rotating parts can simulate Earth's gravity, although I really wouldn't know how it works... Sweet sweet love with every sentient jelly fishes our protagonist finds... Hey, Beer glasses can do wonders...
There's science fiction on the one hand and then there's fantasy fiction set in space on the other, and a distinction between the two is needed before one answers the OP's question. Personally... When dealing with pure science fiction, where the substrate for the rules of the universe is scientific in nature, then only very small deviations from known physics is tolerable. However, with that said, there's still a possibility to insert unachievable feats like time travel and warp speed if you can justify it with a theory (which in itself must not contradict what we already know to be true). Already scientists can achieve teleportation on the quantum level, so it's not too unrealistic to figure out a way to extend such findings to the macroscopic scale, etc. You can still introduce seemingly impossible things in a plausible way, even by using the science nonfiction.
