Google to date is currently the undisputed leader and is placing a high premium on the relevance and quality of its search engine. Most especially since the company it became a public property. In order for them to keep both the users and the shareholders happy, the quality of the results returned in the search engine is giving high importance.
For this same reason, doing the wrong things in the AdSense and other forms of advertisements, whether intentionally or unintentionally, will result in a severe penalty, may get you banned and even have your account terminated, nothing like a good action taken to keep wrongdoers from doing the same things over again.
For those of you, who are currently interested in pursuing a career in AdSense, don't think solely of the strategies that you will deploy to generate revenue. Consider some of the other intricacies' before you actually get started.
Hidden texts, this basically entails filling your AdSense page with texts that maybe to small to read, has the same colour as the page background or using a CSS style sheet for the sole purpose of stuffing them with rich keyword content will generally result in you receiving a penalty award that is given those usually given to those who are concealing links.
Page cloaking, there is a common practice of using browser or bot sniffers to serve the bots of a different page other than the page your visitors will see. Loading a page with a bot that a human user will never see is a definite no-no. This is tricking them to click on something that you want but they may not want to go to.
Multiple submissions, this is basically the process of submitting multiple copies of the same domain, it's always best to stay away from such things. For example, submitting the URL of an AdSense page as two separate URL's is enticing trouble and possible termination. This is why it's always best practice to avoid using auto submitters for those of you who do. Its best you check first to see whether or not your domain as already been submitted to a certain search engine before you attempt to submit to it again. If you see it's already in there, then move on. It'd be pointless resubmitting it again.
Link farms, be wary of who and what are you linking your AdSense to. The search engines know that you cannot control your links in. But you can certainly control what you link to. Link farming has always been a rotten apple in the eyes of search engines, especially Google. That is reason enough to try and avoid them, having a link higher than 100 on a single page will classify you as a link farm so try and not to make them higher than that.
Page rank selling, if you've been online for awhile now, then you'll probably notice that there are many webmasters out there that are selling their PR links or trading them with similar websites. If your caught doing such things, then you could possibly get banned. It's generally okay to sell links and to improve your link popularity. But doing it solely to manipulate page rank is the easiest way to get on a search engines bad side.
Doorways, This is similar to cloaking pages. The common practice of a page loaded with choice keyword ads aimed at redirecting visitors to another 'user-friendly' page is a big issue among search engines. There are many seo firms offering this kind of services. Now that you know what they actually are, try to avoid them at all costs.
Multiple domains having the same content, In case you are not aware of it, search engines look at domains IP's, registry dates and many others. Having multiple domains having the same exact content is not something you can hide from them. The same goes with content multiplied many times on separate pages, sub domains and forwarding multiple domains to the same content.
Most of the techniques that I have outlined above apply to the vast majority of search engines, so it's not exclusive to Google. By having the correct mindset that you are developing your website coupled with AdSense for the human users and not for bots, you can be assured you will achieve the best possible results. Not mentioning that you will also avoid any of the possible penalties liable to you by some search engines and maybe even getting your AdSense account terminated.