These are just basic usability comments. Some of them will apply to the actual wiki code, some to the content itself, and some to the structure of the site.
First, let me say that I am fairly new to using a wiki. We use one for our software development, but I haven't really used one elsewhere. Please forgive me if I don't have the terminology right, or if I don't "get" some basic concepts.
I am starting with the main codex page: http://codex.wordpress.org/Main_Page
The section headings are pretty clear, but there are too many of them, and they suffer from a fair amount of "functional crossover." Also, the main heading names are not clickable, even though there are pages that correspond to them. The little arrows after the names are clickable, but the results are inconsistent.
Various sections are laid out in places that may not be immediately obvious. For example, I wanted to find a list of hook calls. There seems to be no immediate link for that in the main page. Fair enough, so I look for a subheading that may contain this list. I assume that "Advanced Topics" may have what I need. Nothing there that fits the bill, but there is a section called "Plugins and Customization." Maybe that's where it is. I click on that. It takes me to the "WordPress Plugins and Customization" anchor, on the "Advanced Topics" page. It must be here.
Now, here you have a problem. Almost all of the articles here are about *using* and *finding* plugins and themes. They are not about *writing* them. There's "Anatomy of a Template Tag," but that is in the previous anchor, not "Plugins and Customization." As far as I'm concerned, that is *definitely* a topic for writing a plugin.
However, that's not the worst of it. Everything is all jumbled together. You need to create a couple of main headings: i.e. using plugins, and writing plugins. You need an architectural overview (there may well be one, but I have yet to find it.)
The long and the short of it is that I had to click all over to find the minimal amount of information I needed to get started writing a plugin. I think that all the information is there, but it is not well indexed. Basically, you need to have an information architect review the entire codex, and organize the information according to a set of use cases. I only represented a very specific one. It is possible that it is already laid out for more common use cases.
There is a lot more, but I thought that this little anecdote would give a good overview of some of the issues as I see them.