Severe Offenses
There are certain types of harassment that Blizzard considers to be "highly inappropriate." These include racial/ethnic/national slurs or hate speech, language that refers to extreme violence or sexuality, or any insulting reference to sexual orientation. If griefing falls into one of these categories, players can expect a warning, a temporary suspension or a permanent ban from the game. Griefing that involves real-world information about a player or real-life threats is treated especially seriously. In some cases, this sort of harassment has resulted in repercussions involving local law enforcement.
Moderate Offenses
Though some kinds of griefing aren't as serious as the severe offenses, there are certain topics or types of languages that make for an unpleasant gaming environment. While adult language is accepted, players who are overly vulgar or obscene can face account penalties. Bullying, insulting discussion of religions or discussions of illegal drugs can result in a warning or a temporary suspension.
Non-Verbal Harassment
If someone is restricting someone else's ability to play the game, Blizzard GMs may step in. This kind of behavior includes blocking access to an area or an item that another player needs. Since PvP is an integral part of WoW, some non-verbal behavior, while annoying, is not in violation of Blizzard's user policy. A prime example of this is corpse camping -- the act of preventing a dead player from being able to resurrect at their corpse. While corpse camping is one of the most irritating things you will encounter in WoW, this does not count as reportable harassment as you have the option to resurrect at a Spirit Healer or call in your friends for help. You can only report non-verbal behavior if there is no other solution available.
Situational Factors
Blizzard GMs take care to review each harassment report on a case by case basis. All of the factors surrounding the harassment will be considered before a player is slapped with a penalty. For example, public yelling or spamming is treated differently than a private conversation between two players. Harassment within guild chat is generally not punished as strongly, except in cases of severe offenses.