#1 Redesigned Start Page
I like being able to hide the Start Menu and just pin to Start. Reminds me of the Ubuntu Unity desktop where the Launcher can be hidden.
#2 Redesigned Folders
Folders have a more conventional look
#3 Project Spartan
Browser seems to be a little easier to use but FF is still more functional
#4 One little thing it seems MS forgot to put in the ability to sign in with a pin with this build
#5 In file explorer under the view tab/options/change folder and search options I changed File Explorer to open to This PC instead of Quick Access, seems to work a little better for me

Their may be other little things that I haven't found yet but that is the fun of using Tech Previews.