So I went ahead and did the upgrade from gutsy to heron last night. As with past upgrades, I ran into the same problems. Things that worked don't after the upgrade. Here are my pet peeves, but they basically boil down to 'it worked before, and now it's broke' --
1. video driver changes. I've got an ATI video card in my laptop. I use the proprietary driver because the open source driver doesn't (or at least hasn't) supported big desktop mode or clone mode. With the upgrade, I was stuck at 800x600 resolution until I restarted several times and went through the 'fix X-server' procedure. I'll find out if big desktop works tomorrow when I get my laptop back to work. It worked before the upgrade, it should have worked just as well after.
2. wireless changes. I've got a Broadcom wireless card in my laptop. Every upgrade requires that I jump through hoops to get it running again. I did the upgrade over my wireless network, but the upgrade didn't install the parts to make the Broadcom card continue to work. Instead, the card was left non-functional, and to turn on the proprietary driver meant I needed to download another package. Kind of a pain, since the upgrade broke my ability to use my wireless network. So I have to go find a cable and get plugged in. It worked before the upgrade, it should have worked just as well after.
3. xscreensaver removed. I thought just about everyone knows that xscreensaver is better than either gnome-screensaver or kde-screensaver. I had it installed and properly configured, but the upgrade removed it. Not just disabled it, but removed it from my system. At least, the upgrade didn't remove the configuration files, so it was just a matter of reinstalling the package, but again, it worked before the upgrade, it shouldn't have been removed and should have just worked.
Just getting this off my chest. The video and wireless are pretty critical pieces for me, I always put off doing the upgrades because I know I'll have to spend a few hours fixing things that weren't broke.