One of the (theoretical) limitations of WordPress.com the service of free blogs of Automattic is the impossibility of adding advertising to monetize your blog, to obtain income from your presence in the network.
If this is an impediment for you, you may want to take a look at an ßeta service, IdeaFry which has just come out to offer you free blogs of the type ] yourname.ideafry.com supported in WordPress Mu just like WordPress.com but with the peculiarity that you can insert your Adsense advertising codes in the blog.
What does the people from IdeaFry ?, because of 100% of the time that ads are shown on your blog, 10% of the time they will be your ads, the rest, 90% will be your own Adsense banners.  Regarding the service, you will not find anything different from any free blog Wor dPress . There are few themes, less plugins but a special one, which allows you to insert your Adsense code . Also, do not want to choose the location, it comes by default. If you want to see it,.