Google Blogoscoped has surfaced a leaked Google Quality Rater Guidelines which purports to show the ranking scale that internal googlers use to assess the quality of search engine results pages. It appears that the document is gone now from the site that hosted it (hm, time to dig through the firefox cache...)
Aside from some insight into what sounds like a day-in, day-out task of reading search results and noting what's wrong (q.v. sisyphus), it gives a sense of the human level of input needed to put the "right" answer top of page.
Abstracted from the text:
Queries are said to be one of three types: navigational (search for known item), informational (research for a topic), or transactional (desire to purchase). There's a set of expectations for each search type; if you type "IBM" the goog wants you to get to IBM.COM on the top of page one. Pages are quality scored as vital, useful, relevant, not relevant, off-topic, didn’t load, foreign language, and unratable,