UI difference is very huge. They even changed the naming
convention.
There's also the way they handle permissions. In all previous
versions of Windows, all new users by default are administrators.
In Vista, they are downgraded, to the extent that they have to use
Run As Administrator nearly all the time to get something to
run.
Protection of Windows files is now even tighter, you have to bypass
several security levels to remove a Windows file.
There are lots of differences, some are good, some are bad, some
done without thinking. The changing of naming convention is
one.
