Google Quality Raters are humans that Google hires to perform manual checks on Google's SERPS.
Three times now, the guidelines given to these quality raters have been leaked to the public.
In this document, Google defines what they see as the types of spam:
• PPC Pages
• Parked Domains
• Keyword Stuffing
• Thin Affiliates
• 100% Frame
• Hidden Text and Hidden Links
• Sneaky Redirects
But they also list attributes which can make a page an exception to their spam rules:
• Price comparison functionality: Even though the user has to go to another site via the affiliate link to place an order, there is value to have price comparisons right there on the page.
• Product reviews: Pages that provide original reviews offer added value. Items that are commonly reviewed are books, electronics, and hotels.
• Recipes: Pages that provide recipes offer added value.
• Lyrics and quotes: Pages that display lyrics or quotes offer added value.
• Contact information: Pages that provide contact information, especially physical addresses and phone numbers, offer added value.
• Coupon, discount, and promotion codes: Affiliate pages that provide coupon, promotion, or discount codes for the consumer offer added value.
This document should give you a pretty good idea about what Google does and does not want to see in their search results.