I haven't had the time to write a full-blown review of the updates, so here are just a few remarks from a Webmaster's perspective. Scroll down to Quality guidelines - specific guidelines to view the updates, that means click the links to the new (sometimes overlapping) detail pages.
As always, the guidelines outline best practices of Web development, refer to common sense, and don't encourage over-interpretations (not that those are avoidable, nor utterly useless). Now providing Webmasters with more explanatory directives, detailed definitions and even examples in the "Don'ts" section is very much appreciated. Look at the over five years old first version of this document before you bitch ;)
Avoid hidden text or hidden links
The new help page on hidden text and links is descriptive and comes with examples, well done. What I miss is a hint with regard to CSS menus and other content which is hidden until the user performs a particular action. Google states "Text (such as excessive keywords) can be hidden in several ways, including [...] Using CSS to hide text". The same goes for links by the way. I wish they would add something in the lines of "... Using CSS to hide text in a way that a user can't visualize it by a common action like moving the mouse over a pointer to a hidden element, or clicking a text link or descriptive widget or icon". The hint at the bottom "If you do find hidden text or links on your site, either remove them or, if they are relevant for your site's visitors, make them easily viewable" comes close to this but lacks an example.
Susan Moskwa from Google clarifies what one can hide with CSS, and what sorts of CSS hidden stuff is considered a violation of the guidelines, in the Google forum on June/11/2007:
If your intent in hiding text is to deceive the search engines, we frown on that; if your intent is purely to improve the visual user experience (e.g. by replacing some text with a fancier image of that same text), you don't need to worry. Of course, as with many techniques, there are shades of gray between "this is clearly deceptive and wrong" and "this is perfectly acceptable". Matt [Cutts] did say that hiding text moves you a step further towards the gray area. But if you're running a perfectly legitimate site, you don't need to worry about it. If, on the other hand, your site already exhibits a bunch of other semi-shady techniques, hidden text starts to look like one more item on that list. [...] As the Guidelines say, focus on intent. If you're using CSS techniques purely to improve your users' experience and/or accessibility, you shouldn't need to worry. One good way to keep it on the up-and-up (if you're replacing text w/ images) is to make sure the text you're hiding is being replaced by an image with the exact same text.
Don't use cloaking or sneaky redirects
This sentence in bold red blinking uppercase letters should be pinned 5 pixels below the heading: "When examining [...] your site to ensure your site adheres to our guidelines, consider the intent" (emphasis mine). There are so many perfectly legit ways to do the content presentation, that it is impossible to assign particular techniques to good versus bad intent, nor vice versa.
For example "Serving a page of HTML text to search engines, while showing a page of images or Flash to users [is considered deceptive cloaking]" puts down a gazillion of legit sites which serve the same contents in different formats (and often under different URLs) depending on the ability of the current user agent to render particular stuff like Flash, and a bazillion of perfectly legit AJAX driven sites which provide crawlers and text browsers with a somewhat static structure of HTML pages, too.
"Serving different content to search engines than to users [is considered deceptive cloaking]" puts it better, because in reverse that reads "Feel free to serve identical contents under different URLs and in different formats to users and search engines. Just make sure that you accurately detect the capabilities of the user agent before you decide to alter a requested plain HTML page into a fancy conglomerate of flashing widgets with sound and other good vibrations, respectively vice versa".
Don't send automated queries to Google
This page doesn't provide much more information than the paragraph on the main page, but there's not that much to explain: don't use WebPosition Gold™. Period.
Don't load pages with irrelevant keywords
Tells why keyword stuffing is not a bright idea, nothing to note.
Don't create multiple pages, subdomains, or domains with substantially duplicate content
This detail page is a must read. It starts with a to the point definition "Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar", followed by a ton of good tips and valuable information. And fortunately it expresses that there's no such thing as a general duplicate content penalty.
Don't create pages that install viruses, trojans, or other badware
Describes Google's service in partnership with StopBADware.org, highlighting the quickest procedure to get Google's malware warning removed.
Avoid "doorway" pages created just for search engines, or other "cookie cutter" approaches such as affiliate programs with little or no original content
The info on doorway pages is just a paragraph on the "cloaking and sneaky redirect" page. I miss a few tips on how one can identify unintentional doorway pages created by just bad design, without any deceptive intent. Also, I think a few sentences on thin SERP-like pages would be helpful in this context.
"Little or no original content" targets thin affiliate sites, again doorway pages, auto-generated content, and scraped content. It becomes clear that Google does not love MFA sites.
If your site participates in an affiliate program, make sure that your site adds value. Provide unique and relevant content that gives users a reason to visit your site first
The link points to the "Little or no original content" page mentioned above.
Don't trade, exchange, swap or purchase links for ranking purposes
"Buying links in order to improve a site’s ranking is in violation of Google's webmaster guidelines and can negatively impact a site's ranking in search results. [...] Google works hard to ensure that it fully discounts links intended to manipulate search engine results, such link exchanges and purchased links."
Basically that means: if you purchase a link, then make dead sure it's castrated or Google will take away the ability to pass link love from the page (or even site) linking out for green. Or don't get caught respectively denunciated by competitors (I doubt that's a surefire tactic for the average Webmaster).
Note that in the second sentence quoted above Google states officially that link exchanges for the sole purpose of manipulating search engines are a waste of time and resources. That means reciprocal links of particular types nullify each other, and site links might have lost their power too.
<speculation>Google may find it funny to increase the toolbar PageRank of pages involved in all sorts of link swap campaigns, but the real PageRank will remain untouched.
There's much confusion with regard to "paid link penalties". To the best of my knowledge the link's destination will not be penalized, but the paid link(s) will not (or no longer) increase its reputation, so that in case the link's intention got reported or discovered ex-post its rankings may suffer. Penalizing the link buyer would not make much sense, and Googlers are known as pragmatic folks, hence I doubt there is such a penalty.
<speculation>Possibly Google has a flag applied to known link purchasers (sites as well as webmasters), which --if it exists-- might result in more scrupulous judgements of other optimization techniques.
What I really like is that the Googlers in charge honestly tried to write for their audience, that is Webmasters and Web developers, not (only) search geeks. Hence the news is that Google really cares. Since the revamp is a funded project, I guess the few paragraphs where the guidelines are still mysterious (for the great unwashed), or even potentially misleading, will get an update soon. I can't wait for the next phase of this project.
Vanessa Fox creates buzz at SMX today, so I'll update this post when (if?) she blogs about the updates later on (update: Vanessa's post). Perhaps Matt Cutts will comment the updated quality guidelines at the SMX conference today, look for Barry's writeup at Search Engine Land, and SEO Roundtable as well as the Bruce Clay blog for coverage of the SMX Penalty Box Summit. Marketing Pilgrim covered this session too. This post at Search Engine Journal provides related info, and more quotes from Matt. Just one SMX tidbit: according to Matt they're going to change the name of the re-inclusion request to something like a reconsideration request.
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