It’s tempting to pre-determine desirable behaviours.
e.g. no off-topic discussions, no personal insults, no advertising/promotion.
This is known as top-down community planning.
The problem is top-down community planning is risky. How do you know what members want to do? How do you know which behaviours are acceptable for the group a opposed to you? In CancerConnection personal insults would be inappropriate, in 4chan they might be acceptable – even expected.
Aside from the basics (nothing illegal), I’d suggest waiting to see what behaviours the community thinks are approriate. Which behavioural norms emerge? It’s these behaviours you want to encourage. It’s these behaviours that will help develop a stronger sense of community.
If members don’t like to talk about certain topics, put that in the guidelines. If it’s not considered ok to ask someone about any personal details, put that in there too. If most people congratulate someone on a great comment, add that. Just don’t pre-determine these behaviours.
Enforcing normative community behaviours upon new members is a far better option than enforcing top-down desired behaviours upon the community.