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SELinux, native firewall configuration UI, real kernel updates, good availability of packages.
Put simply, since FC5 I never had any stability problems. My only problem with Fedora is definitely boot time. It's snail slow at best. I hope they improve boot times.
Package management is also no more of a issue to me - YUM is now fast and reliable on the command line, though the graphical management for it still sucks because it doesn't give proper feedback when managing big transactions - another area for improvement, though not crucial IMO.
I'm currently testing Ubuntu 7.04 and I can only say that the only thing better than Fedora is boot time - it's at minimal decent speed. Other than that, Ubuntu is not for me. I really want to put emphasis on boot time, since I and others can't get suspend working properly on my laptop and I hate to leave it on (think about wasted energy and global warming).
By the way, to those who call Fedora the RH guinea pig, let me just say that I don't understand why think like this when RH is actually good for the OSS community (lots of contributions) and is actually a decent company that is a reference in how to make proper business out of Open Source. You even have CentOS and other examples that show we're not exactly dealing with evil here.
As for Ubuntu, though it's not my prefered distro, I can't say more than congratulate the project for advancing Linux into user friendliness and novice accessibility!
Now, I can't wait for Fedora 7 to come out and put my hands on it - I can say with confidence that after messing with other mainstream distros (Suse, Mandriva, Ubuntu, Gentoo, Zenwalk, Vector) Fedora is and will be my distro of choice.
Good luck and good work for all the Fedora developers!