Noindexing Images Will Cause Omissions From Image Search & Video Search
Noindexed images won’t appear in Google image search and if a site hosts its own videos the thumbnail image won’t be indexed, meaning that the video won’t be indexed either.
Ensure All Product Pages Can be Crawled With Considered Use of Noindex
eCommerce sites with facets should be careful which pages are noindexed because this may make it difficult for Googlebot to crawl individual product pages e.g. noindexing all category pages. Webmasters might consider noindexing specific facets or deciding that everything after a certain number of pages in a paginated set be noindexed.
Noindex Errors Are Differentiated by Source of URL in the New Search Console
In the new Search Console the noindex errors are differentiated by the source of the URL. Google will assume an error on your side if you submit a noindexed URL, as opposed to finding noindexed URLs found through crawling.
Google Will Remember & Recrawl Noindexed Pages
Noindexed pages will be remembered and crawled by Google. They should be removed from sitemaps.
Use canonicalization Instead of Noindex for Duplicate Content
John recommends using rel=canonical instead of noindex in order to deal with duplicate content in the best way. This way the signals from both page versions can be combined rather than dropping all the signals from the noindexed page.
Nofollow Obsolete When Noindex Already Present
When a page is noindexed, not only will it be removed from the index but also over time all of the links associated with the page will be removed from the link graph so nofollow is made obsolete.
Tag Pages Are Usually Seen as Low Quality
Tag pages are usually low quality and look like search results pages, so it doesn’t make sense for Google to show them in search results and the prefer to show pages with content.
Expired Content Can be Redirected, 404’d or Noindexed
There is no one correct way to deal with expired content on a site. If there is a relevant page that replaces the expired one then you can implement a 301 redirect. If there is no replacement page, then you can leave the page as 200 saying that it is no longer valid and use either a 404 or noindex after a period of time.
A Noindex Reduces Crawl Rate
A page with a noindex tag will be crawled less frequently.