One of the biggest reasons sites are taken out of the Google search index is not low quality content but malware infections. Many sites are hacked/attacked by blackhat users who infect the site with hidden messages (such as links to pharmacy sites, etc). These links are not usually visible to the human eye, but easily detected by Google search bot. If you have cleaned all visible malware off your site but still getting a malware warning from Google, you'll want to try out the 'Fetch as Google' tool. The 'Fetch as Google' tool lets you see how Google sees your site, bare bones. Many links and spam keywords that are being injected into your site's code during a hacking will only be visible this way. Once you use this tool, you'll find out exactly why your site was banned so the offending material injected into your site can be removed. https://support.google.com/webmasters/answer/158587?hl=en Here's some more ways to take malware off your site if it's been hacked. https://support.google.com/webmasters/answer/163634?hl=en