I like the concepts of the bubbles and the fact that it show call execution so well instead of one file at a time. That's really cool, but what I find so amazing is that since they built on top of Eclipse they got much further towards a usable system like having an integrated debugger was a nice surprise to me. 10-20 years ago the gap between an academic project and a finished product would have been much wider. Not to mention they got to explore more ground with the concept. Just shows how open source software enables innovation better.