Popfly, a mashup tool, depends on three things: data that is simple to access programmatically, interesting, and available under terms that enable users to work with it. As with most software endeavors, you can pick two.
Yes, you can program your way out of these, but it's far too hard. Entire organizations such as the Sunlight Foundation are trying to change this, and Lawrence Lessig has proposed what he calls the Open Government Data Principles. And that's great. But it's not enough, because it's not just the government.
Oh, it's not just the booksellers who have terms like that. Any site that makes money off of advertising, for example, is going to have holdbacks in its API terms -- limits on how many calls you can make to the APIs in a given time period or how many results can be returned, or how their brand has to be shown in the resulting mashup, and so on.
As I read a Dow Jones Insight Election Pulse blog post about how much time each candidate spends talking about different issues, I thought, "There's an interesting mashup that I would have loved to build." But the information to create that mashup isn't easily accessible to tools.
Why must good data be so hard to find?