I'll answer each paragraph in order to try to be a "Rosetta Stone" for for Google are broadly labelling as
For example, searchers might get a list of results that all go to the same site.
This technique is colloquially referred to as 'Google Bombing', 'SERP Bombing' or some variation thereof amongst the SEO field.
This refers to targeting a low competition, high intent terms and optimising several pages, including external web properties, for the same goal or call to action.
Having the top half of the first page results return your webpage, blogspot post, tumblr and a youtube video would be an example of 'gaming the results' which is why Google have listed this as a bad practice.
I would argue this is merely content marketing to funnel traffic to your website, not what should be labelled as a
doorway page and this terminology just confuses the community.
However I can see the idea of what Google is trying to solve.
Do these pages exist as an “island?” Are they difficult or impossible to navigate to from other parts of your site?
This is referring to the
landing pages pages developed to convert and may or may not be part of a website's native navigation experience i.e. destination page for an AdWords campaign.
These particularly landing pages are referenced as 'squeeze pages', which are usually long-form copy that describes the pain, dream, solution and call-to-action for the audience.
Some companies have multiple landing pages for specific terms i.e.
/best-blue-widgets, /awesome-blue-widgets that are structurally similar, serve the same purpose and funnel users to
Are these pages made solely for drawing affiliate traffic and sending users along without creating unique value in content or functionality?
I believe this is the principle reason behind doorway pages and its purpose - it is about quality control.
An affiliate marketer can create a single domain for a single search query.
All they do is create content for a
squeeze page, providing compelling copy and provide a link to the product.
Google seems to believe that this isn't providing 'value' to the search user and they will adjust their algorithms based on this purpose.
I cannot possible comment on the SEO consequences until they actually have an update and measure the results based on these changes. I personally disagree with their broad labelling and wished they were much more specific.