Windows WAS familiar
Windows 2000 is familiar, Windows XP is familiar.
Windows Vista and Windows 7 are too different, too flashy and, IMHO graphically painful to use.
There are some good things, like automatic drivers download, better support for sleep and hibernation, you can rename a file without affecting the extension, but...
- The GUI seems to target idiots and people with sight deficiencies (I cannot explain otherwise the need for an 80 pixel tall icon in an alert box that says "Do you want to delete this file? OK/CAncel")
- The control panel is huge and you need to search what to do (problem is that you do not know what to search for)
- Handling files is slow by design. Windows 7 must "discover the files", and think about it for several seconds before actually starting doing the action you wanted to.
- The file manager sucks badly
- UAC is still a pain in the ass with added problems with files ownership
People are generally happy about Windows XP, they are used to it, so why not just build on that without radically changing things?