What guidelines should I keep in mind when making changes to /about?
Make it short
Make it clear
Make it stick
The about page is intended for folks who are visiting your site for the very first time. If you’re lucky, someone who has never used Stack Exchange before will stumble on your site and read it...once.
So you have one chance to influence how they perceive your community, your topic, and the scope of your site. Don’t waste it on minutiae or in-depth explorations of every mistake or misconception that has ever arisen. Instead give a broad overview of what you’re about and why they should care. If they stick around, you will have ample opportunity to introduce them to the finer points later on.
What should go in each section?
Intro section: a short description of the people on the site, the goal of the site, and an overview of the topics.
On-topic section: a list of common topics
Off-topic section: a list of common misconceptions - broad categories that don't work on Stack Exchange in general or this site in particular.
What pitfalls should I avoid?
Most of the folks finding your site are there because they happened upon a good answer while searching Google. They don't care about the history of your community, the massive influx of trolls you faced last year, or why "pineapples" are a thing.
No one is going to carefully compare their question to a massive list of rules or specific subjects taking care to ensure they don't accidentally step outside the squiggly line you've drawn. If you can't explain what you're about in 3-5 short bullets, you have some serious problems.
Also avoid just repeating who you are without bothering to actually touch on the topics you're here to ask/answer:
As your site grows, lots of people will ask lots of questions that are not suitable for your site. But unless a reasonable person would expect some broad category of question to be on-topic - and it isn't - there's no reason to bother listing it here. And lengthy explanations are just tedious.