The first change I noticed was the icon. Calvetica’s icon has undergone a third change and — although I did like the older ones, too — this one screams “I’m a calendar application”, so I can understand why the devs felt the change was necessary.
As for actual app changes: there aren’t really any new features in 2.1. Instead, this update seems to have been about streamlining existing ones. The month view has gotten rid of the arrows for switching years (just swipe on 2010 to switch the calendar year) and the calendar dates are now colour coordinated, which makes Calvetica much easier on the eyes of multi-calendar users. You can also now see the end times for appointments as both text and a red line drawn across the day view, which makes it much easier to find out where you have free time on a given date.
All in all, a solid little update. It’s also gotten me even more excited about the upcoming changes for Calvetica 3.0 (details here).