this whole thread/forum is a wave.
we are all replying to comments made, the same is on a wave.
the power is the real time nature of it, you could literally see me typing this reply if you had the page open as i typed the reply.
if someone posted oh i have one of those for sale in a discussion you could reply to only that person but the overall effect is like a huge instant message thread with branches/comments/pictures/media etc.
take the time to watch the video, yes it's long and at first you will be "ohhh wooo" (sarcastic) but then the power of the system becomes apparent.
if you post to a blog, you can add photos and text as you would do to a blog, you can add others to the Wave (say people you went on holiday with) they can add photos and comments. you can then add a blog robot that takes that thread and puts it on your blog where the world can see it and comment, others collaborate on the wave which updates the blog.
it does have real world impact and could easy replace email systems internal of a company, as most companies use 75% of email as instant messaging and then other tools/apps to create collaboration documents.