A lot of people have commented about the display being fantastic. I personally agree with some people who say its a bit washed out.
I have a black wallpaper and next to my S3, the black just isn't dark enough.
Some of the things i miss from my S4:
my full screen touch wiz calendar widget which shows the whole month and little short descriptions of the items in calendar.
blocking mode - ability to block all notification except the phone ringing or a white list of phone numbers
some of the notification sounds (yes i know i can go download them)
the fact that i need to pull down the notifications bar, and click the menu button, and then click settings to get to settings instead of just menu->settings