We've all known players who seem to think that because they know something, so do their characters. Thus, a character in a fantasy setting decides to build a castle with I-beams and concrete, a character with no knowledge of mechanics decides to build a tank, or a character goes straight for a monster's weak point because the player has read about the weakness in one of the sourcebooks. A close friend of mine describes this as "metagaming" - using out-of-character knowledge to benefit the character. The long and short of it, though is that metagaming of this sort is cheating. It's the same as peeking at the Scrabble pieces or sneaking money out of the bank in Monopoly. It leads to an unfair advantage for one player, which almost always results in the resentment of one's fellow players.
This is not true of every use of out of character knowledge, however. Benign metagaming benefits all participants in the game, and is perhaps the truest mark of a mature player. Forms of benign metagaming include cooperating with the gaming group, recognizing the indivisibility of the party, respecting the other players, and avoiding unnecessary delays.
Cooperating with the gaming group begins before you roll a single die or fill in the first dot on your character sheet. Regardless of the game system, ask your GM what kind of game he or she intends to run and create a character that will fit into that kind of game. This prevents you from building a character whose skills will not be of any use - such as a slow-witted warrior in a game that focuses on courtly intrigue. It helps if you find out what kinds of characters the other players are considering, as this allows you to tailor your character to fit a vacant niche in the party.
No one wants to play a boring character, but it is important to realize the difference between "unique" and "weird." A character who is too distinctive can be difficult to explain. Why would an evil half-troll Monk/Sorcerer hang out with this group? More importantly, why would the good adventuring party let a half-troll join them? This applies to decisions on personality and history, too. As a player you are responsible for every action of your character. Blaming your player decisions on the character is a transparent cop out. After all, who chose her personality and wrote her history? If you create a character who is unable to cooperate with the rest of the party, any problems that character's personality and background cause the party, as well as any resentment the other players might feel toward you as a result, are your own fault.
Certainly, cooperation should continue even after the game begins. This doesn't mean you do everything the other characters ask, but it does mean working together with them when it matters most, such as during combat. A character who never offers the party her assistance not only becomes the target of in-character animosity, she also gains a reputation among the players as dead weight. This does not mean that a character who is not an effective combatant will automatically earn a bad reputation. Each character generally has his own niche within the party, which allows him to shine in certain situations where the other characters would fail. If a character uses her skills to the benefit of the party, her effort will be noted by both players and characters. The important thing is to participate in a way that furthers the goals of the game.
The party is a sacred and indivisible unit because gaming is a group activity and everyone must share a single GM who cannot be in two places at the same time. Sound pretty obvious? Perhaps, but the implication is sometimes ignored when a player decides to strike out on her own. A runaway character is a major headache for even the most experienced GMs. The rest of the party is often forced to chase down their wayward comrade, even if this doesn't make sense for the characters, since the only other option is to let the character leave the group forever, and that means a player sits out the rest of the session or, worse, spends all night in one-on-one game while the rest of the group sits around and plays cards. This doesn't mean the party can't split up into smaller groups for periods of time or that no one can wander ahead of the group to scout or create some mischief, but every character has to come back to the party sometime, and a player whose characters storm off in a huff every other session quickly becomes unpopular with the rest of the group.
The most conspicuous mark of a bad player is his failure to respect the other players. This includes respecting the time and effort they have put into their characters. Players become quite attached to their characters after a few sessions, and there is perhaps no better way to generate a lot of bad blood and ill will than to kill another's character without her permission. Among the most common complaints I hear about bad GMs is their tendency to randomly kill characters simply because they can. It is no more endearing or cool when a player does the same thing.
While any good group of gamers does not want to kill your character or throw her out of the party, it is not reasonable to believe a party will let a companion get away with seriously jeopardizing the survival of the group or with betraying them to an enemy. Nearly as bad is deliberately provoking another character to the point where it would be out of character for him to let you live. Deliberately baiting another player character or seriously violating the party's trust places the other players in a very uncomfortable position and may eventually lead to the problem character's death or expulsion from the party. In no case will it make you look good in the eyes of the other players.
Finally, mature gamers can tell the difference between knowing the rules of the game and wasting the groups' time with technical minutia. Both GMs and players appreciate having a player who can cite rules on demand in their group, since it saves everyone the time and effort of looking them up in the books. No one likes a rules lawyer, though.
Any player who nitpicks over trivial rules, interpreting them in an effort to gain a personal advantage, is only wasting precious gaming time. Few things are as frustrating for a GM as debating a rule with a player while the rest of the group sits around and looks bored. Rules lawyering challenges the GM's authority and denies his fitness to arbitrate the game fairly. It excludes the players from meaningful participation in the game. It is not surprising then that a player's popularity tends to be inversely proportional to the number of times he causes the game to grind to a halt for ten or more minutes with debate over a minor technicality. There are also reality lawyers, players who dispute game mechanics because they do not accurately represent real-world laws and events. Reality lawyers are often even more long-winded and stubborn, but this makes their time-consuming arguments no less of a headache than rules lawyers'.
Whether or not they consciously recognize it, roleplayers of all kinds appreciate the presence of benign metagamers. They ease the task of the GM by saving time and maintaining party cohesiveness. They earn the trust of the other players by respecting the other characters and making themselves useful to the party. In short, benign metagaming is the mark of a mature and courteous gamer, and a group whose players are consistently group-centered are less likely to argue, less likely to lose players, and more likely to enjoy roleplaying together for years to come.