Not having a clear, keyword-rich page meta title and meta description
Not using header tags (H1, H2, etc.) properly
Not optimizing images with proper file names and alt tags
Not having a responsive, mobile-friendly design
Not having an XML sitemap or robot.txt file
Not having high-quality, unique content
Not building backlinks from reputable sources
Not regularly updating and maintaining the website
Using duplicate or plagiarized content
Keyword stuffing, over-optimization.