Vague in, vague out
A prompt like "write about marketing" gives Claude almost nothing to work with, so it has to guess at your audience, length, tone, and purpose, and it will guess something generic. The fix isn't to write a longer prompt for its own sake, it's to include the specific pieces of context a competent human would actually need to do the task well: who this is for, what it's for, what form the output should take, and any constraints that matter. "Write a 150-word LinkedIn post announcing our new pricing tiers to existing customers, in a direct and not overly salesy tone" gives Claude an actual target to hit.
A useful habit is to imagine handing the same request to a smart freelancer who has never worked with you before and doesn't know your business. Anything you'd need to tell that person to get a good result on the first try is exactly what belongs in the prompt. Anything you'd assume they'd figure out on their own is usually safe to leave out.