In 2013, Google removed 350 million bad AdWords ads as part of its efforts to prevent abuse of AdWords by both advertisers and AdSense publishers, although unfortunately they don't detail much about why those particular ads were removed.
In 2012, Google reported killing 224 million bad ads.
The most common reasons ads tend to get removed are for things like products that violates AdWords terms, ad copy issues (such as using all caps or multiple punctuation marks), or having a display URL that doesn't match the destination URL (such as the case of affiliate ads).
Google makes a special mention of their battle against counterfeiters, which is likely in part because of the agreement multiple ad agencies, including Google AdWords, made with the White House last summer to curb the amount of advertising of copyrights infringing and counterfeit goods.
The number of complaints about counterfeiters advertising on AdWords dropped 85 percent in 2012 and another 78 percent in 2013. Any attempts to market counterfeit goods on AdWords decreased by 47 percent in 2012 and 82 percent 2013, showing that it's becoming clear to counterfeiters that they are being thwarted with their AdWords attempts.
Interestingly, the number of advertisers Google removed from the AdWords network dropped significantly from this year over last year's numbers. Google disabled more than 850,000 advertisers in 2012, but that number dropped to 270,000 in 2013. Google attributed this to their safety screenings catching some of these types of advertisers, and going to other "less-secure" advertising networks for their ads.
The role of AdSense publishers within the Google advertising ecosystem was discussed, with Google blacklisting over 200,000 pages on publisher sites. Also, Google disapproved more than 3 million attempts to join the AdSense network, which shows they are definitely trying to tighten up publisher quality within the network. Of the 250,000 publishers removed for various policy violations, 5,000 were due to violating copyright policies, likely from copying content or scraping from other sites.
Something else that is interesting is that there's been discussion on the Webmaster World forums where people are seeing huge uptick in the number of invalid clicks, both from the advertiser perspective and the publisher perspective. We don't know if it's because Google has recently busted some sort of network abuse, if they are tightening up some of the invalid click variables, or a combination of the two. However the display network was mentioned as a definite source of an increase in the number of invalid clicks.
For more factoids about Google and how they deal with both bad advertisers and ad publishers, they created an infographic with more to tidbits: