[Definitely Don'ts]
These things are not allowed:
1) Cheats/exploits (for obvious reasons)
2) Abusive communications (unless you are Lil' Wayne in space you shouldn't really need to)
3) 'Griefing', or the continuous harrassment of specific cmdrs for no reason other than to be a nuisance
Do note that many consider randomly killing people also 'griefing', but that kind of gameplay is really allowed.
[Probably don'ts]
These things are allowed, but often frowned upon:
1) Kill beginners in starting ships while you sport a fancy death machine
2) Being inconsiderate of other players' safety. Eg. boosting through the slot in a massive T9 when smaller ships are in front of you.
3) RP'ing a 'crazy psychopath' without any communication with your victim. In contrast, allying with some theocracy and become some kind of deranged crusader with appropriate interactions, for example, is ok.
4) Kill-stealing: waiting in CZs and RES/NAVs until someone else's target is almost death, then swoop in and kill it off.
[Do's]
Be respectfull and kind towards others. If someone has to be a victim for your RP, atleast make them part of an interesting and unique story that goes beyond staring at an insurance screen. Also understand that the demographics are a bit different here than in the typical FPS game: we have a lot of older folks, some even 70+, and plenty of folks here let their (young) children play. So if, for example, someone is ignoring your comms, maybe it is because you're talking a Romanian six year old boy who doesn't speak English, or a 78 year old woman who doesn't type that fast.
And last but not least:
always assume everyone is insane and out to get you!. I don't know what you're flying, but make sure it is outfitted not for max profit, but max safety. If you're a trader, for example, put a shield, some shield cell boosters and PTD against hatch limpets on there. Make sure you know what to do when a strongr opponent interdicts you. Have an emergency jump location ready, and plot your journey carefully. The vast majority of players are pretty cool, but you'll eventually run into some weird folks. Having prepared yourself is what will have you survive an exciting event, rather than complain online how it was so unfair you got killed.
Be nice & fly safe, commander!