The most basic way to answer the question about what makes Google AdSense so special is like this...

When all is said and done, Google provides any web developer with ads that are relevant to the content of the web page that the ads are being displayed on. They literally read your page before the ads are displayed; get some ads that fit perfectly with the context of your page.

They also give you the ability to not only choose from many different styles and types of ads, but they also allow you to format your ads in almost any way you like. There are choices of text or graphic ads, height and width and they even provide you with a search box that displays ads based on the search that your web site visitor types in, without the visitor having to open a new browser window to get to the Google search page.

If you don't want competitor ads showing on your site, you also have an option of filtering out their ads.

Because you are being paid every time one of your web site visitors clicks on an ad, and your visitors are on your site, it makes perfect sense for the ads you are going to show them to be ads for things that they are interested in.

Not all ad network providers give you such targeted ads, but Google does!

Just imaging visiting a web site that is focused on software development tools and tips and tricks, and the ads were for baking equipment. Do you think you would make much money from your users clicking the ads? I doubt it!