This is mostly about a Desktop OS, but similar issues can arise on server systems, albeit prbably on a less complicated scale.
Some examples.
I have a problem with "audio". Where do I start looking? What is PulseAudio? What is OSS? What is Alsa? Why do I need all those sound servers? how do they interact en how are the different parts configured, tuned and troubleshooted?
I have a Desktop system which I needed to setup with NIS. Now there's some bug/feature in NetworkManager which makes it so that NIS starts before we have an IP address. I needed to disable NM and work with the plaininterfaces config to get it to work. Bu how does NetworkManager exactly interact with /etc/network/ configurations? I can find plenty of doc on the latter, and some info on NM (albeit more scarce), but info on how bothe work together (or not) seems non-existent to me.
I want to configure an external keyboard to use a different layout than my laptop keyboard. I vagely understand that hal can push config to Xorg, but good info on this is hard to find. How does hal work? How does it interact with other parts. Which problem does hal exactly solve and how does it do that? Which parts are solved by hal and which by udev? Are there other processes involved with this? What is hotplug? Just another stanza in the interfaces file, or another building block next to hal, or is it a part of it?
The latter being something that gets changed in Karmic (hal being replaced by something else) is a goog examplke of why it is difficult to get on par with documentation. Stuff changes sometimes in a very fundamental way.
Yes, I know that there's an answer to find on all this if you know were to look, and if you have plenty of time to parse mailinglists, forums, and chat with developers.
But sometimes you just want to get the swork done, and would expect to find good documentation to get the answer. Just like Ubuntu want to be Linux for the end user, it should also help on this level to be more accessible to sysadmins who don't have the time or knowledge of interest to research the inner working of an OS themselves.
I want to admin the systems, not design and implement them. While the latter is of course a good way to do the former, I don't have the time nor the interest to resarch every bit on a low level.