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IMBA (capitalized or not) is an abbreviation for "imbalanced" that is often used within the realm of MMO gaming (especially on message boards) that generally refers to a tactic or ability that is considered overly powerful.
Many Flavor of the Month (FotM) and gimmick builds revolve around one or more "imba" skills that are too powerful to counter without devoting an entire build to the task; as such, gimmick builds tend to roll teams of more-skilled players. Since Guild Wars is touted as a skill-based (not skills-based) game, many players are vocal about imba skills/builds, and bring them to ArenaNet's attention via forum threads/polls.