It's an unfortunate fact that online gaming involves tolerating a remarkable amount of rudeness, vulgarity, poor sportsmanship, and plain old bad manners. Some game communities are a little more mature than others, but you can generally expect this sort of misbehavior in just about every multiplayer game you come across. I understand that people are largely anonymous online, and they can get very emotionally involved in the competition, but that's no reason to throw out any semblance of politeness or decency. I've put together a few pointers on good online gaming etiquette, although I suspect the worst offenders aren't likely to take notice.
Cheating and using exploits is an admission that you can't win without them. Furthermore, you'll find winning a fair fight a lot more rewarding. If winning is really important to you, keep practicing and learn from what successful players are doing. Should the competition prove to be too much for you, finding a different game is a better solution than cheating.
Be A Good Sport
Laughing at defeated opponents, bragging about your achievements, overdoing it with taunts and emotes, all make you look childish. The fact is, even children should know better. An appropriate emote used at the right moment can be a beautiful thing, but when it accompanies every encounter in the game, it turns into that much more mindless spam, which brings us to our next rule.
You really don't need to hammer the chat system with WTS messages to sell an item, nor do you need to use every channel in the game when there is a channel dedicated to trade. If people aren't interested, repeating a message in chat every 3 seconds isn't likely to change that. Wait a minute or two, and maybe you'll start to get through to a new audience.
Spare Us the Caps
C'mon people, this is a no-brainer that has been around since the dawn of the Internet - don't use all capital letters in chat. It's the text equivalent of yelling at everyone, and nobody wants to be yelled at.
You Were a Noob Too
Nobody can pick up a new game and instantly familiarize themselves with all its nuances and details. There are heaps of acronyms and slang particular to each game, and it's absurd to expect people to know these things without a little help from other players. Someone once explained to you what was going on, now you can do the same, rather than simply telling them they're "noobs" and leaving it at that.
A Little Teamwork, Please
If you're part of a team, try to cooperate with your teammates a little. Rambo-style gameplay rarely pays off in a team-oriented game, so don't expect your teammates to be right behind you when you go kamakazi into a gauntlet of enemies. Also keep in mind that actually winning the match is more important than being at the top of the scoreboard.
Don't Be a Quitter
It's amazing how often people will disconnect without a word when they're losing, or drop out when their group encounter isn't going well. If you don't think you can win, surrender. If your instance run isn't going well, try to work it out politely or offer some friendly advice. At times when your group isn't up to the task, explain the problem to others without insulting them. You never know what kind of solutions people will come up with until you give them a chance.
Not Everyone Who Wins Is a Cheater
Just because another player kills you a couple times does not mean that they're cheating. As good as you may think you are at any game, there are likely a lot of players who are just as good or better than you are. If you're as hot as all that, then start playing professionally in LAN tournaments where there is no way to cheat. If you're almost certain that someone is cheating, report them to an admin and let them investigate rather than throwing around accusations you can't back up.
Camping Is a Lame Tactic
Although some games encourage it, camping spawn points and corpses is generally a lame tactic. In many cases it's only possible because of flawed game design, but it's still a weak strategy that players with any dignity don't resort to.
When someone insults you, your best option is to ignore them. If the insults persist, most chat and voice systems have 'ignore' functions to ensure that further communications from them are blocked. Returning the insults typically only escalates the exchange into something you'll regret later.
Enjoy the Game
Games are meant to be fun, and if you can't laugh it off when things go wrong, or forgive a team member for screwing up, then maybe it's time for a break. Most of us have enough stress in our lives, we don't really care to add a bunch of gaming stress to that. When you start taking it too seriously, it defeats the purpose of playing a game in the first place.