1. ᴅᴏ ɴᴏᴛ ɢᴏᴅ ᴍᴏᴅ. Godmodding is when a character features god-like abilities, such as invincibility or mind control, or other unrealistic powers that don't fit with lore. It's also considered godmodding to refuse death in fights or ignore role-players in scenarios in which said role-players are attempting to attack you. Nobody's good at everything; try and keep yourself in check.
2. ᴅᴏ ɴᴏᴛ ᴏᴠᴇʀᴘᴏᴡᴇʀ ʏᴏᴜʀsᴇʟғ ᴏʀ ᴏᴛʜᴇʀs. Being "overpowered" is more or less what it sounds like: giving something or someone an unreasonable, unfair, unrealistic, or unbalanced amount of power, especially when it interferes with the ability of players around you to have fun. This can apply in the obvious ways, like having a character who is invincible or can summon the wrath of a zombie army with a flick of his fingers, but it can also be more subtle. For example, a character who is physically average but has skill mastery of over 70% of all main skills with ease can also be overpowered and put a damper on the ability of those around him to create engaging and challenging roleplay scenarios.
3. ɴᴏ ᴍᴇᴛᴀ ɢᴀᴍᴇ. Metagaming is when a player applies OOC-retrieved information to their IC character, such as participating in a war that you only saw was stated to be happening on a clan's thread on the forums, or hearing whispering because you saw the text, and knowing a character's name because you saw their username. This is the most commonly broken rule of role-play and most infuriating for many.
4. ᴅᴏ ɴᴏᴛ ᴍɪx ɪᴄ ᴏʀ ᴏᴏᴄ. Related to metagaming, it is considered taboo to "mix ic and ooc." That is, players are generally encouraged not to associate information and events that occurred between active, playing characters and events that occur between the role-players themselves. Most often, when players associate OOC information with their IC behavior, whether it be how they percieve another character or actually acting on information they wouldn't otherwise have, it's called metagaming.