On free sites, the placing of the ads can sometimes wreck the design. It's in the top of the main content area. What if you could choose from few different (reasonable) positions? One of them could be the side bar for example. And if you'll choose more of them, you'd get more features (storage space for example)!
Another issue that has been under discussion, is that you can't control the contents of the ads in any way. It's not fun if you have a free site that's against furs and a fur ad is the first thing you see when you enter the site.
Edit: Adsense chooses the ads by the contents of the site so you'd most likely see a fur ad on such a site!
(_|||_) \ - Janne