Today, I installed Android 3.1. I don't find there are any big changes yet. It looks very very similar with my new Tablet, Iconia, which is running Android 3.0. According to Google, it will support more USB devices, such as Game Controller. So far, I tested it in VM. That is the reason, I cannot experience their new features yet. Moreover, there is some major API enhancements, such as new USB host API, the MTP API which is allow the developer to access an external camera and new Real-time Transport Protocol API for streaming. I have not started any development in Android 3.X, so I cannot use them yet. Lastly, another good feature cannot tested the new Hardware-accelerated 2D graphics in my box . Even the VM supports it, but I have not a graphics accelerate card at all. Generally, I can find some UI improvements, but I cannout find much other stuff. I need to have a real Android 3.1 device, not just a VM to test!

Reference:
Android 3.0 Platform Highlights from Google