The editorial standards of what's in the user-contributed (wiki) documentation aren't high, but that sort of thing seems endemic to wikis everywhere. The other thing that's missing -- and this is a complaint I'd extend to Ubuntu's on-board help as well -- is that many things really need screenshots to properly illustrate what's being talked about. The discussion of the desktop panels, for instance -- one of the things that's relatively distinct to the desktop in Linux distributions that use GNOME -- has barely anything at all to illustrate what's being talked about.
When it comes to individual applications, the picture's pretty appealing: Most of the community-supported programs for Ubuntu have good, consistent help. OpenOffice.org has some of the best help of the bunch (and it better!), as does Firefox. -- even if these last two applications use entirely different interfaces for how they access their help repositories, neither of which seem to have anything to do with Ubuntu's own built-in help system. This is something I see a lot of on Windows, too, so it's not exclusively a Linux thing.
I can think of a few other things I'd like to see in this department -- one of them, ironically enough, being something that's turned up in Windows as of late. There, if you open the event log, any individual event ID can be cross-referenced against Microsoft's knowledge base, and the functionality for this has been expanded even further in Vista. Granted, Microsoft doesn't have entries for everything, but the few times I had a show-stopper of a problem, I was able to look up its log entry and get a working solution for it in under five minutes. It definitely beats Googling for cryptic error reports -- which stinks no matter what platform you're on.