Not so long ago we discovered a simplified version of Nautilus which seems to miss Ubuntu Karmic after all so the only way to get it is through it's PPA. But still, one of the most requested features in Nautilus must be the dual-panel view, which gives you the ability to work in two directories at one. Sure, you can use tabs, but a dual panel Nautilus would be really useful.
KDE has Dolphin which has this feature but for Gnome users, there is only Gnome Commander and I for one would like to see this in Nautilus.
Holger Berndt's (via @ ubuntu brainstorm through gnometips and ubuntu life - thanks guys!) implemented this into Nautilus so everyone can enjoy the dual panel view.