I was a Fedora user until Fedora Core 6 but the second Ubuntu distribution was a really nice experience so I made the jump (fast packaging system, huge repo, etc).
Since 9.04 (or just before) I have been annoyed by the notification system NotifyOSD, I find disturbing to blur (or hide w/o compositing) the message on mouseover and the popup doesn't provide contextual actions. The Ayatana project get Ubuntu away from the upstream development (many patches to integrate libindicate, etc).
The Fedora Core 12 releases notes caught my attention and so many of my GNOME heroes (such as Alexander or Owen) work for Red Hat so after one test I  jump back. After one month, I'm still an happy Fedora user
What I like:

• yum is fast (for my needs)
• Eclipse packaging (CDT, valgrind, systemptap)
• all stable updates of GNOME
• good integration of PulseAudio with the kernel
• recent version of Empathy
• meta-package for development
• PackageKit
• rpmfusion is very useful to collect bytes from the dark side or for the freeworld (eg. font rendering)

What I don't like:

• wrong French keyboard by default
• easy_install is provided by setuptools-dev package not setuptools
• no sudo user by default
• the kernel updates doesn't update the default GRUB entry to the latest kernel
• SElinux (enabled by default on my desktop)
• no /etc/ld.so.conf.d/local.conf by default (for /usr/local/lib)
• gtags.el isn't in Emacs site-lisp directory
• you need to install control-center-extra to have access to Windows settings (that really sucks)!