I know characters are not supposed to know the exact location of traps or npcs in a quest, but is knowing, for example, that some chosen have isaac's missile storm, or that chosen favor electricity traps (provided you already fought chosen) metagaming?

If your character has encountered them, this is not metagaming. Clarification: PMing someone OOC and telling them to prepare for foes counts as such. As a rule of thumb, if your character knows something, it's fair to speak of it IC.

If you as a player know of something your character does not, it is metagaming to speak of it IC.

Trying to add a little, you have to approach quests as if you've never done that specific quest before, but it is okay to apply your character's general knowledge that relates to a situation he or she has encountered previously.

For example:

Bad: "Peter! There is going to be an electric trap surrounding that chest up ahead, but if you approach it carefully from behind you can avoid triggering it."

Ok: "Be careful friends, I've seen Chosen set up electric traps before, so let's be careful and search these areas"

-- Don't ruin it by going directly to the place where you OOC'ly believe there is a trap to flag, or, conversely, do not intentionally avoid 'searching' an area where you believe a trap exists that you wish to avoid.