Once again, I can understand your frustration with having a computer that is malfunctioning. However if you think that hardware diagnostic programs are going to be of much use to the average person, I think you are mistaken.
Once again, realize that OSX comes with everything that Sys9 did but with many things necessary under Sys9 no longer being an issue. Further there are diagnostic programs that are standard on OSX that weren't in Sys9, many that run in the background.
Looking at XP here at work, it comes with only the following programs: Backup, Disk Cleanup, and Diskdefragmenter. The latter is highly crippled and most suggest getting a 3rd party application that does the same thing. Further that tast, as several mentioned, isn't necessary under OSX. Disk Cleanup is basically also an XP only type of problem. Backup is about the only exception. However even there a special backup program is only necessary for XP because of the Windows registry. In OSX that simply isn't a problem. Most programs you can simply copy the whole folder and move it where ever you wish without problems. In Windows you CAN'T do that. If you move it most programs stop working. Further most programs store everything in a database called the Windows Registry which is very hard to deal with. In OSX simply copy ~/Library/Preferences (or the whole ~/Library) and you'll get all your application settings.
So even the backup program really isn't necessary. You can drag and drop files to perform a backup. Apple supplies a backup Applescript and there are plenty of 3rd party backup programs. For serious backups you should be running Retrospect or one of its competitors - that's true on Windows, Unix or OSX. I actually do think that a simple backup program for the Mac would be a good idea. However I also think that writing one yourself with Applescript or the shell is an even better idea. That way you know exactly what is being backed up and when.
Don't get me wrong. I'm not saying OSX is perfect. I think an interface to a scheduler (like cron) ought to be provided. I do think a simple backup program for simple users would be a good idea. (Perhaps part of iLife?) Perhaps improving the bootup diagnostics a little and providing a little more feedback would be helpful. Heavens, I can think of a half-dozen fixes for the Finder that would improve things for everyone. (Apple - PLEASE improve multithreading)
My problem really is that I don't think any computer fits your criteria for "ready for prime time." The sad fact of the matter is that a small percentage of systems will have serious problems. Apple, according to statistics I've seen, does better in reducing this than most PC manufactures. However it happens. It sucks if you are the one it happened to. However if, instead of trying to fix the problem yourself, you'd simply taken it to where you purchased it, you'd have a working system by now. It is under warantee so I'm not quite sure why you are still complaining about there not being a good diagnostic program. There was a simpler solution staring you in the face. It is akin to the guy who is lost complaining about poor street signs but refusing to ask for directions.