1. It's not polite to start a battle with your biggest attack. Crippling your opponent when the action is just getting started cuts the RP short for both of you.
2. It's also considered cheap to stun someone, then follow up with your most powerful attack.
3. While you are allowed a parting shot on retreating opponents, it's not very nice to purposefully try to knock them unconscious with a high-level attack. (Or use a stun on them.)
4. If a player is stunned and a round passes without anyone attacking him/her it's considered 'the fair thing to do' to +combatroll him/her OOCly to remove the stun flag.
5. RP damage realistically. If you're half-dead, try to look like it. If you're close to losing consciousness you probably won't be moving around much or striking out with powerful attacks.
6. Use +allow or +miss if the situation warrants it. The same goes for +selfhurt.
While some actions, such as firing your heaviest attack repeatedly or using a stun on a retreating enemy, might seem logical because they would be 'In Character' for your character to do, these actions are frowned upon because they violate the spirit of the game, which is to have fun and play fair. In some instances, realism and being 'In Character' must take a backseat to fairness.