Not being a writing expert, I'm going to say something about fight scenes seeing as that was what I got most of my positive feedback for. This should to some degree work for all types of fights (swordish, gunnish, fistfightish) I'll keep it short don't worry.
1. Keep it realistic!! Unless you are trying to make a character superhuman in some way (which can easily be the case) keep to stuff that is possible of a typical human. There is a difference between a skilled warrior and a guy out of Dynasty warriors (back in the PS2 day) who nobody swings at and who can just kill others by swinging his weapon with no skill at all.
In short, In the words of Sensei Ali G "Keep it real!"
2. Make sure the fight makes sense. If three opponents attack, make sure either three opponents are properly subdued or pwned, or else your characters are. If an opponent (or one of your characters) are kicked to the ground or taken down non-lethally, then they are not properly subdued and should either be finished off or should get up again. I know this is sounding like a video game manual but its how fights generally work. Unless there is a magical element present, the fight should make proper physical sense.
3. If you are unsure if your "fight choreography", or so we'll call it, makes sense, go through the moves yourself and see what works and what doesn't.
4. Your character isn't always going to get the first attack in, especially against multiple enemies. However this can work to your character's advantage as an avoided attack can leave the opponent off guard and primed for a counter attack.
5. The amount of violence you include depends on your target audience. Don't write a basic, easy to understand "they all lived happily ever after" book if you have the guts and brains of the bad guys being splattered over walls in every fight scene.
