QUOTE: “Search engine optimization is often about making small modifications to parts of your website. When viewed individually, these changes might seem like incremental improvements, but when combined with other optimizations, they could have a noticeable impact on your site’s user experience and performance in organic search results.” Google Starter Guide, 2008
In particular, the Google web spam team is currently waging a PR war on sites that rely on unnatural links and other ‘manipulative’ tactics (and handing out severe penalties if it detects them). And that’s on top of many algorithms already designed to look for other manipulative tactics (like keyword stuffing or boilerplate spun text across pages).
I think ranking in organic listings is a lot about trusted links making trusted pages rank, making trusted links making trusted pages rank ad nauseam for various keywords. Some pages can pass trust to another site; some pages cannot. Some links can. Some cannot. Some links are trusted enough to pass ranking signals to another page. Some are not. YOU NEED LINKS FROM TRUSTED PAGES IF YOU WANT TO RANK AND AVOID PENALTIES & FILTERS.
Another example when the “nofollow" attribute can come handy are widget links. If you are using a third party's widget to enrich the experience of your site and engage users, check if it contains any links that you did not intend to place on your site along with the widget. Some widgets may add links to your site which are not your editorial choice and contain anchor text that you as a webmaster may not control. If removing such unwanted links from the widget is not possible, you can always disable them with “nofollow" attribute. If you create a widget for functionality or content that you provide, make sure to include the nofollow on links in the default code snippet.
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
Being ‘relevant’ comes down to keywords & key phrases – in domain names, URLs, Title Elements, the number of times they are repeated in text on the page, text in image alt tags, rich markup and importantly in keyword links to the page in question. If you are relying on manipulating hidden elements on a page to do well in Google, you’ll probably trigger spam filters. If it is ‘hidden’ in on-page elements – beware relying on it too much to improve your rankings.