What changes in version 2.1 compared to 1.5?

[eluser]antoniog[/eluser]
As the demand for the title of your post, what are the substantive differences between the two versions?
At first glance I noticed that in version 2.1 application folder is outside the system folder and classes, Model and Controller (and I assume others) now have the prefix CI_.
What other changes?
You know how to get information about changes between the versions?