I ran into a problem right off the bat: the installer wouldn't boot. The kernel just got stuck partway along and froze. It turns out there is a bug with the TPM driver on certain hardware, which causes a timeout error:
tpm_tis 00:0a: tpm_transmit: tpm_send: error -62 tpm_tis 00:0a: tpm_transmit: tpm_send: error -62 tpm_tis 00:0a: tpm_transmit: tpm_send: error -62
This error can be worked around by disabling timeouts in the tpm module. Unfortunately, even though this issue was known before Fedora 12 was released, it wasn't fixed, nor was it even mentioned in the release notes. Sigh.
The touchpad also wasn't working properly. After install, you can't tap it to click on things. The simulated 3rd button when you click in the top right corner is broken. I googled and found that the touchpad needs to be configured in the Gnome control panel, however I'm running KDE and it didn't seem to have a touchpad applet. Turns out it wasn't installed;
yum install kcm_synapticsfixed that and I was able to enable the functionality that should be on by default.
Fedora 10 introduced a new graphical boot for systems which have appropriate video drivers; in Fedora 12 nVidia hardware is included using the nouveau driver. The nouveau driver, however, is unfinished and it corrupts the display often (but the graphical boot is nice). In the end I had to install the binary nvidia driver because flash video was freezing the machine.
Speaking of boot, on boot Fedora starts the GDM Gnome login manager. Annoyingly, KDE's switch user feature is still broken unless started from KDM; you try to switch users and instead the screen locks and that's all that happens. No error, no warning, just a broken feature. Switching to KDM requires editing a config file.
Installing software through kpackagekit is needlessly difficult. Finding the right package to enable mp3 playback in amarok is an exercise in futility. Amarok itself provides no indication of what is wrong; it just tries to play a song and fails, skipping the song and going to the next, with no error message or anything. If all your music is mp3s amarok will just keep chewing through the playlist, happily not playing anything.
Also, for the first time in about 4 years, my printer doesn't work out of the box. Drivers for every possible bizarre input type or video card or sound card or network card or HPC IO controller or satellite launcher are installed, but my printer's drivers were not.
Finally, because I wasn't feeling punished enough, I also tried upgrading my HTPC to Fedora 12 from Fedora 11. I have no idea how badly things are broken because, to be honest, the Fedora 11 installation was pretty fuzzed to begin with. But at least the upgrade installed and nothing seriously broke.
On the HTPC the KDE installation is missing some pieces because I can't log in using KDE as my desktop; instead I get no error message but am booted back to the login screen. Some KDE apps don't work properly. Also annoying is the fact that I can adjust my fonts to make things readable on the TV but only one of GNOME or KDE sees those settings, the other doesn't, but my menu contains either KDE apps or GNOME apps but not both. An average user would be pretty annoyed that they want large fonts, and 90% of their desktop uses the right fonts, but there is a hidden, mysterious control panel that asks you the same exact questions you already answered in some other program, but you have to answer again or else 10% of your apps will look wrong. Now, this is a Gnome vs KDE issue, but the fact remains that Fedora 11, installed fresh, and then 12 installed as an upgrade overtop, left me with a hybrid Gnome/KDE system, where the only graphical software installer uses tiny fonts because it's a KDE app and I'm stuck in Gnome.
If I get the HTPC working properly again using MythTV I am going to never upgrade it again.