I find that it's incredibly hard to describe to people exactly what Wave is and why they should use it. In practice, it's very simple, but there's no quick and easy way to describe it. What seems to amaze people the most is actually something that's been around for a long time: real-time transmission of text.
First of all, while I know it is a preview, it's just extremely slow when working in waves with more than a few blips. Of course, I'm sure this will improve soon enough (before an actual public launch). It's just something I wanted to point out.
I've actually seen one thing changed in the one day I've been using the service. This morning, if someone were to edit a blip, it would show that it had been edited by another user, but you would have to dig into the timeline to actually see the changes. Now, they are clearly displayed as highlighted text. It's a very nice addition.
Gadgets and robots also need a little work. Each gadget currently has its own button on the toolbar. While that's great if you only have one or two, it could easily fill up the toolbar with clutter. There's also the fact that robots are added into your Google contacts, so if you don't want a bunch of Wave robots scattered throughout your contacts, well... you'll have to play around with it a bit. A separate section for robots would be nice.
I will say this: I'm not a developer, so the potential Wave has for me at this point is very limited. However, once I get a few invites, I'll be able to actually invite people I know and see what happens. It's not quite ready for the public, though. It's slow, it occasionally crashes, and at this point it would not be a very fun experience for anyone who just wants to get something done. Google Wave does have huge potential, so keep an eye on it. Just don't expect it to be perfect right away.