The inbox manual seems fine to me, as long as the digital manual is easily accessible.
The in-device manual was somehow not obvious to find (I accidentally ran into it when looking at the contents of the drive from my PC). It ought to have a link on the first view with the Get Started applet.
The in-device manual is missing a few things, such as how to edit or delete radio streams, the limitations of the video player (nothing over 848x480 gets played), a picture of the special characters map (so it's easier to find by just browsing the manual).
Info on Manage views needs to added that states that turning of a view deletes its apps/widgets contents permanently (so turning a view back on results in an empty view apart from background image).
The manual ought to have a reference section pointing to maemo.org, maemo.nokia.com, ovi.com and nokia.com for further info as appropriate. In fact on complex topics like the video coder support, links could be added in the relevant manual section to the appropriate support-subforums.
It could also do with a link to the latest version of the manual (preferably on the first page), so people can trivially get the latest if OS updates warrant any changes.
I'd also like to have the pdf version on the phone instead of just the html version.
Multi-language manuals in the same pdf are however not my thing (the American English one comes with Spanish in one file).

The above naturally all from memory from when I just got the N900.

The community doesn't seem particularly well suited to check manuals, since most members are way too advanced users to notice the obvious missing stuff that somebody who is upgrading a phone after 3-4 years without being familiar with the support forums would be scratching his head over. At any rate, it'd be a bit too late for the on-device manuals anyway.