Wordpad and Paint both use the ribbon from office now, paint is actually pretty awesome now and wordpad can open docx files from microsoft office and odt files from openoffice and other programs. The calculator does unit conversions and stores your equation history.
It's got a better backup system, more efficient use of multi-core processors and some fancy new power saving technology that greatly reduces power consumption if you're one of those people who leaves their computer on all day.
Stuff is stacked in the taskbar and there's a quick preview window. You can right click on your browser button in the taskbar, and depending on your browser, it will give you the option to visit recently closed tabs, book marks, your most frequently viewed pages, etc.
Documents are organized in libraries. It's a good system.
If these are features you might want then you should consider upgrading. Eventually DirectX1337 will come out and you won't be able to enjoy the dynamic water splashing effects or whatever they add to it because DirectX1337 will only run in Windows 7.
If you have a valid .edu email address (or your parent/neighbor/friend) and Windows Vista you might as well upgrade now anyway since it's only $30.
