But what really killed it isn't being talked about much: Network effects.
A network effect, or network externality, is an economic term referring to when the value of a product or service increases the more users it has. Imagine being the first person to own a fax machine: if there isn't anyone to send faxes to, it's not very valuable. It's the reason why websites like eBay and Facebook seem to dominate instead of existing among many competitors. (In Japan, Yahoo! Auctions was an early mover, and is the dominant player; and Google's Orkut service is the preferred social network in Brazil.)
Google's reason for killing Wave is that "Wave has not seen the user adoption we would have liked." And all of that can be traced to how Wave was introduced. After a tremendous presentation at Google I/O, there was real buzz and anticipation for Wave. It was a prize to be able to get a Google Wave invite. Eventually, I got a Wave invite too. I signed up as fast as I could and then... I did nothing.
I signed into Wave and sent a Wave to the person who invited me. Only, they weren't on Wave much, so it took them a while to get back to me. I wasn't on Wave much after that, because there was no one to talk to. I think you get the idea. In some cases, causing an artificial shortage of supply can be beneficial. But that is the exact opposite of what you want if your product is subject to network externalities.
Ironically, Google seemed to learn this lesson too late, and then had a much misinformed launch for Buzz. Buzz is Google's... well, I don't think Google is really sure what Buzz is supposed to be. It's kind of like Twitter, but more like Tumblr. Tumblr is a light-weight blog, without the Twitter character limits, and primarily works as a link dump. Tumblr doesn't allow comments, while Buzz does. Anyway... Buzz's launch failed because suddenly everyone with a Gmail account had a Buzz account, which just so happened to reveal information about who you email the most. Whoops.
But Buzz isn't catching on and partly that's because Buzz has to worry about the network effect in a different way: Users have finite energies to dedicate to social networking. Facebook fills one aspect, and Twitter picks up the slack by being different in some key regards. Getting a Facebook account is valuable, because so many people are on Facebook. The same is true with Twitter. But these are active users, which provide you with a reason to go on and stay on. Knowing many people with a Buzz account isn't the same if they aren't active on Buzz. And many people disabled their new Buzz accounts anyway, due to its tendency to over-inform you about comments made by friends-of-friends to a post you didn't even comment on.
Google should have been much more open about letting people use Wave. It should have allowed anyone with any-email-account-at-all to automatically have an account. Here's how it should have worked from Day One: If you wanted to send someone a Wave, you would use their regular email address within Wave. That would have then sent them an email with the text of the Wave, plus a link to view the Wave itself. At that point, you could opt-in to Wave and choose to have further updates sent to your email address or to set up reminders to check it only when major updates to the Wave have been made. Seeing the advantages of Wave, users would stick around and start sending out their own Waves.
Instead, not even Gmail users got a Wave account. Hardly anybody got a Wave account. And those who did found it to be just-another-website-to-check. Had Google even done the cursory Gmail-integration that Buzz has, and made Wave part of Gmail, it might have seen more success.
***Although, actually, it wouldn't. There were too many other problems:
1) Users didn't want character-by-character typing; it was a flaw, not a feature. IM programs (remember when they were standalone?) could have done this a decade ago, and there's a reason they haven't supported it. Some users don't even like the "is current typing" messages in some IM systems.
2) As mentioned above, there was poor integration with Gmail. Having to check yet another website just isn't productive. [Message to Google: Please integrate Voice with Gmail, for the same reason.]
3) There were major bugs. I tried Wave for a project with three people. I wanted to use it as a wiki and discussion system. It sounded like the perfect application for it. But it couldn't even scale. It had numerous server and client side bugs and poor conversation threading support. It was also far less wiki-like than that Google I/O demo. Really, it was like a different product entirely.
4) None of the extensions seemed to work. Making a new poll, for example, wasn't intuitive or possible. And then "Add gadget by URL"? Really? That's how you make productive users? Instead of showing them a page of possible extensions, and populating the quick access list with a dozen actually useful gadgets, you wanted users to enter a URL?
5) Over half of the screen was by default non-Wave content. I could imagine the default working only for those few folks with very large monitors and the habit of having fully expanded browser windows. Each time I tried Wave, I would have to click to minimize my "inbox," just so I could see content. I bet there is something about that in the usability literature, because any single extra click a user has to make ("it's just one click!") seriously effects their overall experience.
I'm glad that Google is an engineering company, in the sense that they gave a bold idea a fair chance-- there wasn't a real business case, it just seemed like an interesting artifact to make. I just wish it was an engineering company that knew more about economics 101.
Update: Steven Levy's book In The Plex covers some really interesting stories about Hal Varian, Google's chief economist. They do have tons of good econ skill when it comes to auctions!