I want to get some of my friends into roleplaying by running a self-contained single-session game with them rather than an actual campaign opener (for lack of a better term, I'll refer to this sort of game as a 'one-off'). However, I have no experience in writing this sort of scenario, and while I'm confident that I can take the techniques I use in campaign scenarios and try to use common sense to figure out how to apply them to a one-off, I thought I'd ask for some advice from people who have already done so.
How is writing a good one-off different from writing a good campaign scenario? Any advice for what I should put more effort into (quick immersion in the action) and what I should diminish or remove entirely (exploring the characters' goals)? What makes for a good one-off that might not be initially apparent to someone who has never written one before? What should I expect to be different when running a single-scenario game with new people and how should I react to/plan for it?
It is more than fine if you use examples from systems in your answers, but please try to focus on principles that apply to any system as opposed to the intricacies of a particular one.