My favorite part about Windows 7 is search. I can now quickly find any file no time. The ability to find individual emails is particularly awesome. Couple that with Jump Lists and pinned items on the taskbar and I never find myself dragging windows over and under each other to find something I just had open.
Jump Lists are a list of items you had recently opened in a particular program that is brought up by a right-click on an application in the taskbar, whether it's "pinned" to it, or otherwise. A list of recent files/websites/media or what have you that have recently been opened by that application pops up, and you select where you want to go from there. For instance, if I have Excel open, but minimized or behind another window, and I want to go straight to a spreadsheet I opened earlier in the day, I right click the Excel icon on the taskbar, and a list of a dozen or so docs that I've recently looked at in Excel pops up for me to select from. It saves time from the old way of having to open up/maximize Excel, then closing windows until I get to the file I want. Works the same way with web browsers, which keeps my million open tabs easy to keep track of.
Two other things I especially like about Windows 7 are the ease of file sharing with my roommates using HomeGroup (password-protected), as well as switching seamlessly between printers at work and home.