> A kind of best-practice report on how to use Wave productively
I think that, right there, is why Wave failed. It needs "best practices" to work well.
The tool on its own did not improve productivity. It needed discipline and training to learn how to use the tool, and there was a large gap between what was possible and what was useful or productive. By many people's accounts, once you cross that gap it becomes useful, but for anybody who doesn't self-identify as an early adopter, it's a non-starter.
Compare Wave to email, or to IM. There are people who are good at email and people who are bad at email. There are people who just don't get the medium, and are bad at communicating with it, and who could benefit from learning some email etiquette or best practices. But you know what? Even without being good at email, they can still use email effectively. Even when the medium is not being used to its full potential, it's still more efficient than other media. The same goes for IM.
With Wave, this is not the case. If you throw a bunch of people at Wave, the first thing that comes out is an unstructured, ad-hoc clusterfuck. It doesn't become useful until you standardize a process on what gets its own wave, what info goes where, and so forth. Learning to be productive in the tool is an additional step beyond learning how to use the tool. And whatever you decide, isn't enforced by the tool itself, so if someone doesn't "get it," that can put information in the wrong place or disorganize stuff, and make everything confusing again.
Compare this to email, where there is an existing workflow defined by the Compose, Reply and Send functions. Put the text in the box, other person reads the text, and each email is a line item in Outlook. In short, all the organization that you have to invent in Wave is already present in email, and it's enforced by the tool rather than a gentleman's agreement not to make everything into a clusterfuck.
Wave lets you make your own rules, but it also makes you make your own rules, while email comes with its own set of rules built-in. For some people, being able to make your own rules is useful because email's are sub-optimal, but for the majority of people, email is Good Enough, and the benefit of making your own rules is worth neither the cost of having to invent your own process, or the friction of having to maintain it by fiat.