> Take that guarantee away, and someone could design a reputation currency where participants could reject payments from disreputable participants. In which case accumulation of units does not imply the long-term ability to use them; instead, there will be a long-term incentive for participants to maintain a good reputation.

+1, considering that the empowerment of individuals to issue the currency themselves creates a trust graph that should be enough to mitigate many of the downsides he brings up. Here's my attempt at social, reputation cryptocurrency (but it's currently being rewritten from scratch): https://github.com/sunny-g/whuffie/blob/master/ABOUT.md