It wasn't for me but i would like to meddle a little if you don't mind
1- It's a difference in philosophy. Having to place and then remember locations of where you put your files is an analogy of a physical filing system. That analogy was once needed, but now we don't actually need to spent time managing our files, that's a task the system can and should provide to us, and that actually gets in our way of improving the file usage...
Onenote MX does a great job exposing this new concept: When you open it the first time it searches all your skydrive files looking for onenote books... They also open one themselves... You are never presented with a dialog box to look for the files yourself nor do you have to chose a location where to store new notebooks, and everything just works.
I believe this old system needs to die, and they may have to break it at some point, and to be able to do that they need to gradually start changing the way both the system and the user understand the new file management system... Since they are already adding a new interface that breaks a lot of old concepts i guess they felt like it was the perfect time to starting breaking this one too...
3 - I guess the idea behind this separation is that you don't
have to cycle through... How often do people actually need to look at the same time for an app, a file or a setting? All the other times you would be presented with less search results from the category you are actually looking for... Sure, having the option would be ok i guess, but for me it's actually an improvement since i get more results from the group i want, not having to click the category to show more results like it was before...
4 - Not sure i got that one, but there is a desktop tile by default, that takes you to the desktop...