There's a bunch, but most of it comes to the following: 1. Mis-matching declarations - e.g. a meta robots tag that conflicts with a HTTP Header x-robots-tag for example 2. Missing declarations - missing title tags, canonical tags, etc - not often critical - but not ideal either 3. Missing redirections from no-www/http - often overlooked 4. Open-graph images that 404 5. XML sitemaps that 404 or reference 404 resources 6. Mis-configured SSL/TLS headers and more...