Scrub
The video game is what it is. It defines what you can do and what you can't do. In competitive games, the rules exist as written into the game world. Sometimes, this leads to an over-powered character. Sometimes, it leads to a Game Breaker.
Some competitive video game players accept that some characters/races/etc. are more powerful than others. When they're being competitive, they play one of the characters/races that are on the top tier and ignore the rest.
Some people do not hold to this, and may develop house rules designed to bring what they feel is a game imbalance back into balance, so that there is more variety (after all, if a game has 20 characters, but you would clearly win using 2 of them, then those 2 characters are not worth playing). Essentially, a self-made game patch. This is all well and good.
However, it can go too far. When it does, you have the Scrub.
A Scrub is a player of a competitive video game who adamantly believes that their "house rules" should apply to everyone to promote their view of "fair play". If a scrub sees a move or strategy they don't like or can't beat, they "ban" it and/or complain that anyone who uses it is cheap.
The mistake the Scrub often makes is making up rules too soon. The Metagame can often turn an apparent imbalance on its head. A lower-tier character can become a higher-tier one, or vice versa. Or something that seemed initially very unbalanced can be countered with time and effort at learning the tactic. The Scrub circumvents this by simply banning the practice without making a good faith effort in actually getting around it with the in-game rules.
The Scrub will often consider the tactics that he bans "Game Breakers". However, what constitutes a Game Breaker varies from person to person. Within a particular skill level, a game can be broken in that a simple repetitive tactic can lead to success. A more skilled player could stop it, but if it requires a great deal more skill to stop the tactic than to perform it, then it can easily appear to be a Game Breaker.
What ultimately makes the Scrub undesirable isn't the rulesetting; it is the attitude. Players will often agree to refrain from using certain moves or from selecting certain characters so they can have fun without investing a lot of time into learning the game. The Scrub, on the other hand, believes that his way is the only proper way to play the game and refuses to acknowledge that the game could be more fun any other way.
Scrubs may also put down others who use certain characters/teams/etc. for simply using them like everyone else is using in order to win. In other words, they don't like it when people use something that everyone else is since it's not "original" and will even go as far as accusing these people at sucking at the game for not using anyone or anything besides the top tiers. In Collectible Card Games, this is often referred to as "net decking" because they believe the player took their deck idea from a successful deck they found on the internet.
Combined with the Noob, a Scrub can quickly turn into a God Modder.
Compare and contrast with "Stop Having Fun!" Guys, who are about as annoying to say the least. The Scrub is the antithesis of the "Stop Having Fun!" Guy, although one could argue that the two are very similar -- "Stop Having Fun!" Guys demand that everyone use the most efficient and powerful choices (with failure to do so considered proof of inferiority), while Scrubs insist that everyone follow their personal house rules (with failure to do so considered cheating). Both make the mistake of taking things very seriously, both are more than willing to start a Flame War rather than let the other exist in peace, and both are eager to assign anyone to the side of their enemies who doesn't agree with their style of play (if you're not a Scrub, you must be a Stop Having Fun Guy, and vice versa).
On the other hand, the term can be used by players to indiscriminately insult players they deem unworthy, or simply players they don't like. It gets thrown around so much in World of Warcraft community that in most of the cases, it has lost its meaning.
This article and this contain some of the most thorough investigations of the "scrub" phenomenon you can find anywhere.
Not to be confused with the TV series Scrubs, and also not to be confused with a guy who thinks he's fly and is also known as a buster.