Ragequit
To ragequit is to quit when things are not going your way. This phenomenon is common to all team-based games where there is no penalty for leaving, and the term ragequit is not limited to Guild Wars. People who ragequit often do so because they believe that their team has no chance of success, though it is also possible that they are infuriated at other members of the team or themselves.
In arena PvP, ragequitting is commonly seen in the Random Arenas. Players will often just leave if there is no monk on their team, or if there are more monks on the opposing team. The Dishonorable Combatant System was created in part to discourage this type of ragequitting in the Random and Team Arenas. It is somewhat common in Heroes' Ascent and Alliance Battles when the teams are pick-up groups (PuG). In Heroes' Ascent, specifically, PuGs will often break up after the first defeat instead of retooling and taking another shot at the Hall of Heroes; members who ditch the team after the loss are thus sometimes said to ragequit.
In Guild versus Guild, one occasionally sees players leave the game right before their Guild Lord is killed. While strictly part of the definition, this form of quitting doesn't doom a team that wasn't already headed for a loss. Such ragequits are generally an expression of disappointment in their team, or of disgust at losing to a perceivedly inferior guild. Monks often quit in this fashion as they are often accused of inadequacy, though warriors will also ragequit if they are not healed during an overextension, as will lead strategists if they are disappointed or angered at their performance.
In PvE, ragequitting is common among PuGs. Many players have rigid opinions about how certain missions and professions should be played, and quit at the first sign of deviations from the norm.