I think wikis (if used) need to be seperate than documents in a chart like that. Wiki (meaning quick, as in edit) documentation is meant to be ever-changing, while documents imply a final product, or locked status (which can be achieved on a wiki with editorial controls, but defeats the purpose of it as well). Documents may also imply a physical product, like a manual or pamphlet.
Also recognize that wikis generate discussions (on the back end usually, and sometimes only involving minutae as opposed to content). Blog posts also generate discussions via the comments feature. I recommend that the discussion forums and the commenting capabilities of your blogs be integrated - a blog post is really nothing more than a very long initial post in a discussion forum. Unless of course it's a micro-blog, like a twitter post, then it can be shorter than an initial discussion forum post!
Lastly, a bookmark/url can also be seen in the same light as a blog post or forum post.
Pondering an idea and asking a question can be seen as the same thing.
While the metrics you describe such as shape, formality and common uses (for which there are always exceptions) are perfectly legitimate, how *useful* are they when compared to such metrics as permanency/ephemeral, direction of communication [one-way (announcement/rhetorical), two-way (discussion/question)], and intent (generate discussion/answer question/create product)?
I'd pick a couple dimensions and graph the capabilities of each type - showing overlap like a Venn diagram.
Josh, thought I'd comment as someone who has been using Jive SBS since 2008 and who is our most prolific user of our 13,000 employee community at Bupa.
The easiest way to answer the question of how to do something on Jive is to take the off line analogy and apply it online, all Jive does is allow one to do the normal social interactions one would do in a meeting room or conference and do these online. So to answer the question in detail imagine you have all these people in your community or group you've invited and they are sitting next to you in a meeting room.
Blog post - opening statements of an idea or observation to set the scene and to invite comment and discussion around a theme
Discussion - Can be a quick, 'hey have you seen this' or 'how about this, anyone know anything?'
Docs - Good for summarizing information or a like a whiteboard with post it notes to collate the valuable bits of info of discussions in blog posts / discussions / etc
If you follow the rule of thumb of what would I do offline and apply it to Jive you generally can work out the answer pretty easily.
Hope this helps.