There are a lot. I think that the Client Access Model is really going to change how interaction with SharePoint will occur. I can see a fully Windows based SharePoint in the near future. However, my favorite change is the new notification area of the User Interface. For those that are not familiar with it, compare it to the pop-up that shows when a user logs into Live Messenger or Communicator. There is a pop-up that shows for a small amount of time telling you that someone you care about has logged in and then it disappears. It is fairly unobtrusive but quite useful.
The notiifcation in SharePoint 2010 acts pretty much the same way. You get a little message showing up inside SharePoint that you can act on or ignore. The best part is you can write code to show your own messages (I am not sure what messages are there by default). There are many, many ways to use this. Some that I can think of off the top of my head is a user login notification, new document notification, new task notification, and so on.
I am sure as I use SharePoint more and more there will be other new features that will catch my eye but this is my favorite….so far.