The main improvements in Windows 7 are small UI changes (basically the dock and start menu come to mind). Most other UI changes in Windows 7 are annoying at best and really degrading to the user experience at worst (please try to find the display power saving settings in 7, and compare that to XP).

Under the hood, Windows 7 is mostly identical to Vista (After all, it’s a minor upgrade from vista – from version 6.0 to 6.1) and is really bad: the networking protocol was made slower and more complex, the graphics performance was made more complex and slower, more security (basically targeted against the user, not for him) makes the kernel slower, and its not even more stable: the main improvement in Windows 7 over Windows XP is that now the BSOD automatically reboots your computer instead of waiting for you to press CTRL-ALT-DEL