The biggest one I keep seeing: treating meta titles and descriptions as an afterthought. Developers will spend weeks perfecting an app, then write a 10-word meta description that's just the site name repeated.
The second thing — and this one really hurts small business sites — is no structured data. A local business with zero schema markup is invisible to rich results. It takes maybe 30 minutes to add basic Organization or LocalBusiness schema and the difference in click-through rate is measurable.
For the Core Web Vitals point: LCP is usually the killer. Most devs don't realize their hero image is loading uncached from a CDN with no fetchpriority="high" hint. One attribute change, massive improvement.