Slight visual updates, quick settings, paginated app drawer+ movable tabs. Those are the main differences.
Sense 2.1 is also referred to as GingerSense.
Also, cut-n-paste improvement. You know those little "select text" markers?... they are now much easier to operate. At first I didn't like the change, but now I love it. The first long press gives you a choice of "select word" and "select all". Notice, no "select text" choice. So, you zero in on the word where you want to start selecting, it becomes highlighted with only one marker to drag until you get to the end of what you want to select. Much better and more accurate than the old way.
Also, the Mail app, when in the "All Accounts" view, will display a MOVE choice (like to the Junk Folder) with the first press of the menu button. It used to require Menu | More | Move | on Sense 2.0.
Also, calendar app, much easier to set an appointment time. The little slot machine counters now paginate at 15 minute marks so you can quickly find any 15 minute increment. I can't tell you how frustrating it was to to me to have to play with those stupid wheels to get the proper time in there!
Also, new Downloads app to take you directly to the download directory. Even if you don't have a file explorer installed,
Also, (this *may* have been there before) when viewing the "All Accounts" Mail inbox, you can easily search for a specific message with the Search capacitive button -- as well as using the Mail menu system.
Also, dealing with Exchage meeting requests is better.
Also, faster.
I'm sure there are more differences from Sense 2.0 and 2.1, but these are the ones I noticed that made the most difference to me.
-Frank