The mashlet UI support OOTB is simply but has limited features, right? e.g., it's not possible if the users want to define some drag and drop features with JavaScript?

We bundle with EXT libraries (www.extjs.com) to create an OOTB experience. And since our APIs are open, you can build the UI in many other toolsets too; examples include BackBase, Flash/Flex, Silverlight, Java, JavaScript/Ajax, and Ruby.