It seems like a number of problems (erosion of privacy, abuse of advertising channels, etc) would cease to exist if users were charged some small sum for the use of Web services that are now ad-supported.

As someone who's utterly naive when it comes to building a Web property, I'd like to understand why this idea is bad/hard/crazy. What is a typical set of eyeballs really worth on a high-traffic site? Are paywalls the kiss of death?