I'm not saying marketing (as an industry) is bad or even writing content for marketing purposes is bad. Think more in terms of those articles you've read and thought, "hang on - has that actually said something useful?". I.e. articles that are just thinly veiled pitches.
Re: measuring quality. Can but speculate, but I don't think it would be too tough for Google to pay thousands of quality raters over the years to produce data that could be folded into search results. Less about "quality" as we may think of it - perhaps "relevance" is a better term to use? I speculated as much earlier in the year (http://www.fusednation.com/search-engines/google/google-panda-getting-your-rankings-down/), although that article is *very* speculative and still has stuff in it that has since been disproven (so pinch of salt if you read it).
10 out of 100ish, so about 10%.
To elaborate on what I was saying above, the articles that were removed were thinly veiled SEO efforts, compared to the rest which (while, not particularly well written), contained well researched, useful information.
Working on the theory that Google at some reviewed my site using quality raters, the articles removed were (in the context of competing results) slightly relevant / useless, whereas the rest of my site may be considered useful / relevant for it's targeted terms.
I think that right there is the important distinction to make. Marketing intention, etc aside, if you want to rank for "webmaster forum" you need to be an actual webmaster forum, and not just have an article (however well written) on the topic of "webmaster forum".
Going off topic slightly, I think the integration of quality rater information is why we've seen brands getting more weight over the years. It's less about Google favouring corporations and simply just normal users favouring brands when rating SERPs.
How is Google measuring quality?
Who says it's Google doing the measuring? I think we all might change the way we approach websites if we assumed one person out of one hundred to visit our site is acutally submitting feedback directly to Google that impacted our rankings. Just a thought. :)