4.0 Has made a lot of changes. Including some keymap changes. To help people out here is a video explaining the more common changes: Watch here
If you prefer to read all the changes then Read here
You can also go through the Release notes on the Blender Site which includes a video by CGCookie's Jonathan Lampel.
If you just want to read the release notes without all the fancy videos and demos then you can Read Developers Release notes.
Hopefully this will address some of the confusion for those who choose to use 4.0 or like me 4.1. I like submitting bug reports. It makes me feel like I'm helping in my own small way.
