Not Apple's fault - read the workaround
>To remedy, users were instructed to rip a technology known as APE, short for
>application enhancement, from the Leopard's maw. The process is too gory for us
>to print here, but suffice it to say that, among other things, it had users use a
>command line to purge files with names like "Enhancer.prefpane" and
APE, short for "Application Enhancer" is a third-party app (look at com.unsanity.ape.plist - this signifies that it's an app called "APE" from unsanity.com) that has deep and potentially unsafe hooks (/hacks) into the system, to modify applications and system functions. It's not really a surprise that it screws up when an entirely new version of the OS is dropped in there in front of it (It should check this, and not load...)
http://unsanity.com/haxies/ape/ and refer to http://www.unsanity.com/products/compatibility where it is listed as "not compatible with Leopard".
That is why I recommend an "Archive and install" not an "Upgrade install". You just don't know whether your previous apps will fully work with the new system ESPECIALLY if they are hacks like this.
Search for the post by "Chris Mcculloh" and read all the replies to it.
And from reading that thread, nearly everyone who did an "Archive and install - where the installer puts all the previous system files in a folder called Previous System and installs a fresh new system, instead of overwriting in place, had a successful install.
Reading any lower, I see that someone else reports another workaround for those ones that still don't work. Look for the post by indiekiduk. He says that there is another problem with ShapeShifter - a third party theming tool that again, messes with system internals.
And further down, 4F!ller recommends removing iScroll - a third party trackpad driver.
So as far as I've seen, the vast majority of these issues come from third-party software that has been installed. This isn't Apple's fault, and Apple should not have to support these bits of software. These pieces of software SHOULD check to see what OS version they are running on, and demand that they be updated if the software is installed. It's kind-of like expecting engine bits from your old car to work in your new car without damaging it (if we must bring out the inaccurate car analogies). That said, IMO, Apple should make Archive Install the default, not the in-place install.
"but where does the Blue Screen Of Death come from, shouldn't it be a gray screen with a transparency window in the center or something?"
You know that blank light blue screen when you shut down? That shade of blue. My guess is that it's stopping critical system processes from starting up, and leaving the system with no apps running (not even the login window).