Possibly my most tricky style guide yet, this installment pertains to word choice and writing style. Even I'm aware that this is a dicey topic and very subjective, but if anyone has some suggestions, please throw them in the comments!
First off, some really basic tips: flavour text (the italicized text found at the beginning of region, landmark, Willowdale, PC and NPC pages) should be in second person, while everything else should be in third person. Spelling and grammar should be decent, but hey, we all know there's no spell checker and we have at least 3 people meticulously going through pages for spelling anyways, so don't sweat it.
Some more nit-picky things: capitalize 'Pathfinders' when using it to refer to a group containing one or more PCs. When you make a link to another page (especially things like quests), try not to hyperlink the whole sentence; keep it to 2-3 words.
A slightly less concrete style choice: Overall, we've been trying to keep this wiki as though it's been referring to a story world and not a DnD game, at least explicitly. To that end, when you're writing content, try to avoid phrasing things in a meta-style, especially when talking about events, people and monsters. I know there are some places where it's necessary (Card Items), and some places where it makes sense (talking about the spells a person knows, or their class level), but try to keep stuff like that limited.
That's all that I can think of for the moment. To the comments!