There is a WebmasterWorld thread with almost 100 messages already named Reporting violations, the Ethical Dilemma. It has a discussion of which types, the characteristics, of sites that run AdSense should be reported and which ones should not be reported for AdSense violation. As you know, often there is a gray area when it comes to pushing the line with the AdSense TOS.
For one, some folks may feel a bad or guilty feeling reporting some of these sites. On the other hand, some may not be sure if they have stepped over that line. The rules are simply not 100% clear and they cannot be, or should they be? Heck, look at Google Domain Parking, if those sites don't look worthy of reporting and they are approved by Google - you get my point.
The folks in the forums are struggling back and forth with questions like these.
Forum discussion at WebmasterWorld.
