Planning trips, wave is invaluable. I'm an avid rock climber, and I used wave to constantly update people on trips, and used interactive maps to plan them (the map feature is arguably Wave's best gadget).
I also use wave to keep action item, backburner, and reference lists for myself and my business partner.
Lastly, I attempted to use wave instead of a forum on my website. It would've been great, but no one fucking knew what it was. In my opinion, wave's failure was a marketing one, not technical.
Lastly, it's more convenient for collaborative creative content generation than any other tool. The slashdot community, believe it or not, is the opposite of Wave's target market.
I hope Google has a replacement brewing for my purposes.
"There is nothing new under the sun, but there are lots of old things we don't know yet." -Ambrose Bierce