1) When the semantic web didn't emerge, I got excited by embedded microformats as a way that many of the same benefits to be achieved in terms of computable knowledge.
2) I was always optimistic about P2P tech until I realized that we live in a world dominated by centralizing forces, both governmental and capitalistic. P2P can't exist in a recognizable form without some centralization that allows law enforcement or capitalistic gain from the service it attempts to provide.
In both cases, I underestimated the control that governments could exert, and the power to capture dollars and mindshare that centralization due to corporate profits would entail.
You see that in the web itself. The demise of RSS, the demise of the personal website, the reemergence of walled gardens. We're back to AOL, albeit with three or four players taking the role of one. What are hashtags but scaled up AOL keywords? What are Facebook groups but scaled up AOL forums?
The web of those dreams will only exist in the margins and never at scale. Like ham radio or hobbies outside the mainstream that don't scale.