I think the problem with Wave, is that you needed a group to really make it worthwhile. The rollout kind of crushed it. Gmail worked with ANY other email client. I use wave for all kinds of things. I use it to track customer's websites. Adding content, assets, etc to the wave. If I am working with someone else, I can add them, and we both have the same information. We can also chat, and collaborate about what we plan to do.
I use it to keep recipes I use frequently. I will tend to get into a rut, and not know what to make, I go to my "go-to" recipes that are fast, and easy to make.
When I was inviting people, I created a large wave with tools and links on how to use it. As I invited people, I added them to the wave.
I use it for bookmarks between work, and home... but they also work elsewhere, as well, say at a friend's house.
I use it for managing tasks. Again, global availability. I love wave, and am REALLY sorry to see it go. I really like Buzz (as a Twitter replacement) and I don't see that working either. *sigh*