Google is looking for a “website that is well cared for and maintained” so you need to keep content management systems updated, check for broken image links and HTML links. If you create a frustrating user experience through sloppy website maintenance – expect that to be reflected in some way with a lower quality rating. Google Panda October 2014 went for e-commerce pages that were optimised ‘the old way’ and are now classed as ‘thin content’.
A poor 404 page and user interaction with it, can only lead to a ‘poor user experience’ signal at Google’s end, for a number of reasons. I will highlight a poor 404 page in my audits and actually programmatically look for signs of this issue when I scan a site. I don’t know if Google looks at your site that way to rate it e.g. algorithmically determines if you have a good 404 page – or if it is a UX factor, something to be taken into consideration further down the line – or purely to get you thinking about 404 pages (in general) to help prevent Google wasting resources indexing crud pages and presenting poor results to searchers. I think rather that any rating would be a second order scoring including data from user activity on the SERPs – stuff we as SEO can’t see.
Comparing your Google Analytics data side by side with the dates of official algorithm updates is useful in diagnosing a site health issue or traffic drop. In the above example, a new client thought it was a switch to HTTPS and server downtime that caused the drop when it was actually the May 6, 2015, Google Quality Algorithm (originally called Phantom 2 in some circles) that caused the sudden drop in organic traffic – and the problem was probably compounded by unnatural linking practices. (This client did eventually receive a penalty for unnatural links when they ignored our advice to clean up).
QUOTE: “Content which is copied, but changed slightly from the original. This type of copying makes it difficult to find the exact matching original source. Sometimes just a few words are changed, or whole sentences are changed, or a “find and replace” modification is made, where one word is replaced with another throughout the text. These types of changes are deliberately done to make it difficult to find the original source of the content. We call this kind of content “copied with minimal alteration.” Google Search Quality Evaluator Guidelines March 2017
Make it as easy as possible for users to go from general content to the more specific content they want on your site. Add navigation pages when it makes sense and effectively work these into your internal link structure. Make sure all of the pages on your site are reachable through links, and that they don't require an internal "search" functionality to be found. Link to related pages, where appropriate, to allow users to discover similar content.
QUOTE: “Returning a code other than 404 or 410 for a non-existent page (or redirecting users to another page, such as the homepage, instead of returning a 404) can be problematic. Firstly, it tells search engines that there’s a real page at that URL. As a result, that URL may be crawled and its content indexed. Because of the time Googlebot spends on non-existent pages, your unique URLs may not be discovered as quickly or visited as frequently and your site’s crawl coverage may be impacted (also, you probably don’t want your site to rank well for the search query” GOOGLE
Websites that have extremely negative or malicious reputations. Also use the Lowest rating for violations of the Google Webmaster Quality Guidelines. Finally, Lowest+ may be used both for pages with many low-quality characteristics and for pages whose lack of a single Page Quality characteristic makes you question the true purpose of the page. Important: Negative reputation is sufficient reason to give a page a Low quality rating. Evidence of truly malicious or fraudulent behavior warrants the Lowest rating.
QUOTE: “Supplementary Content contributes to a good user experience on the page, but does not directly help the page achieve its purpose. SC is created by Webmasters and is an important part of the user experience. One common type of SC is navigation links which allow users to visit other parts of the website. Note that in some cases, content behind tabs may be considered part of the SC of the page.” Google Search Quality Evaluator Guidelines 2017