In my personal opinion, a half-rolling release model would be a great idea. I want my base system(xorg/kernel/gnome or kde) to be as stable as possible. But why would anyone need to wait 6 month or use some PPA to get the latest version of Firefox/Chrome/GIMP/Whatever? I was taking a look at Chakra (a KDE-oriented distro with Archlinux roots) a few days ago and found their half rolling-release model idea to be extremely good. I hope to see something similar in other distros in the future.