If a game is being run by a GM, Storyteller, etc., the rules and game play are completely up to them. Please play it their way.
Breaking Game Rules, AKA Cheating, AKA Mistakes Were Made: Please do not intentionally break the rules, when gaming. MORE importantly, please be patient with the 99% of us who are not perfect at teaching or playing games. The group may want to house rule a mistake (or misinterpretation) as being in effect for the entire game. Many games have rules that are interpreted differently by different players. The game's designer(s) should be the authority, but sometimes even their clarifications are fuzzy. Do your best, but above all, have fun.
If you are new to gaming, there is an etiquette that takes time to learn, that makes the hobby fun, or at least not annoying. Here's a few helpful practices:
Try to include players, by not demanding your own seat at the table. Scheduling a game is good for a planned experience, but if the game is less important, try to break up into smaller groups, to include people who just walked in. Once a game has started, it's reasonable to turn someone away. Until then, think about it from the perspective of the other person, on the sidelines. Almost no one shows up to watch other people play.
Before starting a game, try to be aware of the type of game, and the time it may take to play it. Let others know this, so they can decline, rather than leave before the game is finished. Remember to ask them, if there is any doubt, or don't be upset if they do leave early. Likewise, try to inform others of your limit on time, or your preference for other games. It's usually better to not play a game, than to play half of a game. However, be aware of the time allotted for your game, at a convention. If it can't be finished before the next game starts, please find a way to end it early. Often, players are committed to later games, and this is to be expected.
Try not to complain. If you don't like a game, now you know, and can avoid it in the future. People do not want to hear constant negative remarks about the things they like. Two hours of annoying remarks makes it worse for everyone. If possible, see if they can start over without you, if it's only been 5 minutes, and you can't bear it.
Attitude, in general. If you only play board games once in a while, you probably don't know the vibe of seasoned gamers. Believe it or not, it doesn't matter who wins, and who loses! They are there to enjoy the shared experience, and see how well they can do. Even in a tournament, it's not all about winning. Relax. Smile. It will be o.k. If it's not o.k., ask yourself who is upset, and not having fun.
Analysis Paralysis. We all play at different speeds. Some people take much more time than normal, to analyze their moves. If that is you, please don't do that. Get it in gear, or you might become known as that A.P. player. This is especially important at a convention, where the games need to be finished, or even abandoned early, so the next game at that table can begin.
Taking care of game materials. Be aware of how you handle boards, cards, etc. Sleeved cards should not be riffle shuffled. If you don't know what that means, let someone else shuffle, or show you how. Be gentle with games you don't own. Keep your hands clean. Keep food and drinks away, if it's not your game. Cards do not have to be "snapped" when played, and tokens do not work better when rolled around in your hand like poker chips. Just be aware, and the games will last longer. You can offer to help put a game away, when finished, but the owner may prefer to do it in a certain way, so let them. Don't pick up their board. If it cracks or falls apart, you were the last one to touch it... Also, check the floor for any stray pieces.
Hygiene. It has been a common complaint that gamers don't always keep theirselves smelling clean. Let's fight that perception by taking breaks after many hours of gaming, and if necessary, taking more than one bath / shower a day. Some of us sweat profusely, no matter the temperature, so deodorant is a must. This is obvious, but a helpful reminder.