One reason Ripple will likely fail in my opinion is this concept of allowing USD to be shifted "anonymously" among accounts. At least their marketing spin said you could do this. The recent laws about transfering USD around will mean this will likely be restricted or stopped.

I'm actually interested in what their source code will bring to light because their marketing claims are pretty big. And from the early structures I looked at it looks like it's going to be less efficient than Bitcoin their transaction engine. But wait and see!

You make some good points about the web of debt and such that is also another issue.