In any work of fiction you must bring your reader with you. You must convince them to believe in the reality of the unreal parts of your story, the term most commonly used for this is the "willing suspension of disbelief." If your audience doesn't believe in your story, you've lost them.
Speculative fiction has a double charge against it on this front in that it is both unreal in the particulars of your characters' stories (i.e. fiction), and in the setting (the world of the fantastic). So the spec fic reader has to work doubly hard to suspend their disbelief, which means the spec fic writer has to work doubly hard to earn that suspension.
Because of this, the spec fic writer has to be even more careful with details than the general fiction writer and ground the non-fantastical and fictional details very firmly in reality. Understanding and writing believable combat is very much a part of that since combat is so often an important aspect of the literature of the fantastic. So is making sure that your fantastical details are internally consistent. And getting your science right. And those latter aspects of world building are the next thing I want to talk about, though it'll have to wait a day or two.
Thoughts? Questions? Criticisms?