My initial reason to try Fedora Linux was that a lot of good things (glibc, SELinux, systemtap, gdb) are mostly developed by Red Hat and put into Fedora. So it felt natural to try Fedora Linux to see the state of things. Now after more than a year with Fedora I am back to Ubuntu/Debian. For some simple reasons:
yum is ultra slow. When I try to search a package it decides to update the package database over the network, it is slow to start, it is slow to carry out simple operations. The other is that the various databases on the system are quite big and take a lot of space.
My upgrade to Fedora15 was hosed. Not so much to complain about as the distribution is not meant to be released the next week or such. So I was experiencing integration issues (e.g. setroubleshootd causing SELinux denies which will start the settroubleshootd), systemd not logging to /var/log/messages.
With Fedora15 I also had the opportunity to try GNOME 3.0 and in many ways it is promising, I don't think it is ready for end users yet though.
The biggest issue with Ubuntu right now is to get SELinux up and running, their policy modules are quite outdated.