OK, I should explain what happened to me. It may not be what has happened to you, but I should document it someplace so that if it happens to someone else Google will find it.
I have used Ubuntu for several years, but I decided it was time to move on and learn about other distros. I had tried several live CDs over the years and was always favorably impressed with Fedora, but I decided to start with Debian Testing, mostly because I love Debian package management. I bought a new hard disk for my laptop and put the Jaunty disk on the shelf for "just in case" and as a source of data and config files.
After a couple days of installing apps and configuring things the way I wanted them I decided it was time to reboot. And after I logged in the gnome-panel and metacity were gone. I had turned off display of the desktop, so I couldn't even create a launcher to get a terminal window open. I spent a day googling with another computer before giving up. However, I was suddenly hit with a bunch of work that required the computer to be working, so I put the Jaunty hard disk back and forgot about moving on for the time being.
Three weeks later I tried again. And again I started with Debian Testing, except that I rebooted more frequently. And sure enough, toward the end of the second day I lost the gnome-panel and metacity again.
I was so disgusted with Debian Testing by then that I installed Fedora 12. And in the morning of the second day I rebooted and discovered that I had lost metacity again, although this time gnome-panel was still running. Note that each new distro install was completely fresh, reformatting the new hard disk.
I quickly gave up and installed OpenSuse 11.2. At the end of the first day I rebooted and discovered that I had lost metacity again.
Now, each of those three distros use the latest Gnome. I never had anything like this happen with Ubuntu, but Ubuntu does not use the latest Gnome, not even on Karmic. Quite naturally, I blamed the problem on a bug in Gnome.
At that point I installed OpenSuse 11.1 to get an earlier version of Gnome. I used it for a couple of days and never lost gnome-panel or metacity. But I really disliked it. Package management was a mess.
So I decided to wipe it out and install Fedora 11, which also uses an older Gnome. I was moving right along, installing, tweaking, and having a fun time. And then on the fourth day I rebooted and discovered that I had lost metacity gain.
I should add that I had tried many troubleshooting efforts. One of them was to create a new user. And I discovered that the new user still had gnome-panel and metacity. Therefore I knew it had to be something in my user configs. But each new install was on a reformatted disk, and I never copied any configs from any other distro, with the exception of the .openoffice.org file from my Jaunty drive.
I also tried deleting .gconf, .gconfd, .gnome2, and .nautilus folders. They were regenerated and I lost my settings, but I still had no metacity.
It finally occurred to me that I needed to figure out what was different between my user profile and the new user's profile. I decided that an easy way to do this would be to pollute the new user with my Gnome configuration dot folders, one folder at a time, rebooting each time. All that happened was that the new user gained my desktop settings; the new user still had metacity.
And then I tried polluting the new user with my .local folder. Bang! The new user lost metacity,
In the process of polluting the new user with my configs I had always renamed the new user's folder by appending -original before copying my folder to the new user. So I ran diff on the two folders. It was hard to read the results from diff, but eventually I nailed the culprit. I had a file called "metacity.desktop" in .local/share/applications that was not in the -original folder. I tried renaming it, but Nautilus refused to let me rename it. Eventually I renamed it "metacity.desktop.old" from the command line as root.
And when I logged out and back in I had metacity back.
I have no idea what rogue process created that evil file, but I'd sure like to know so I can file a bug report.
So far I have rebooted several times and metacity always loads automatically. And the metacity.desktop file has not reappeared. (Knocking on wood.)
I have asked a number of local Linux-Gnome users if any of them have such a file. No one one has it.
That's as much as I know. I hope my story will help others who run into the same thing.