Ubuntu and Fedora are arguably the most popular Linux distros out there today. They both make a huge impact on the Linux community release after release, but are somewhat opposite philosphies at times. Fedora, the RedHat-sponsored community project is adamant of RPM packages, while Ubuntu is based on Debian and therefore uses DEB packages. Fedora maintains that RedHat corporate environment vibe to it, more like a specialised distro, the perfect choice for developers. Ubuntu, on the other hand, based its strategy around creating “Linux for human beings”, a friendly desktop environment that is accessible to all kinds of users.
One of the common things they had up until their latest release was the default desktop manager of choice. Both offered GNOME as their main option, along with fairly good implementations of KDE and other alternatives. Unfortunately (or fortunately, depends on how you look at it), that is no longer the case. Come their April/May releases Fedora will become the flagship implementation of GNOME3 and GNOME Shell, while Ubuntu will step into uncharted territory with its own developed Unity interface.
It is clear that many will reject both and stick with other distros which remain loyal to classic GNOME2.x. It is highly unlikely that many Fedora users will reject GNOME Shell to embrace Unity, though, but not so much that some Ubuntu users shall decide to ditch Unity to use GNOME3 and GNOME Shell instead. Because Fedora is currently the best and most current implementation of GNOME Shell, many are already attempting the switch.