The first part of the talk addressed the issue of users that don't know how to optimize their queries for Google.
One thing that does distinguish users is the difference between a novice search user and an expert user of search. Novice users typically type queries in natural language while expert users use keyword searches.
Example Novice and Expert Search User Queries
NOVICE QUERY: Why doesn't anyone carry an umbrella in Seattle?
EXPERT QUERY: weather seattle washington
NOVICE QUERY: Can I hike in the Seattle area?
EXPERT QUERY: hike seattle area
On average, it takes a new Google user 1 month to go from typing novice queries to being a search expert. This means that there is little payoff in optimizing the site to help novices since they become search experts in such a short time frame.
Google actually ignores or pays little attention to prepositions, conjunctions, articles, so people realize that it's faster and more efficient to only list the keywords. But there are already search engines like hakia that want you to use full sentences to take advantage of the relationships between words. Maybe users shouldn't adapt to search engines' limitations, but search engines should become smart enough to understand our requests.
Update: The video of Marissa Mayer's keynote: