Constraints can also encourage creativity, because people have something to push against. Larry Niven (science fiction writer) claims that constraints make a story (although he arguably writes puzzle fiction; and puzzles are defined by constraints).
I've seen a similar idea in niche marketing, where you specialize on a segment instead of spreading yourself thin to please everyone. Does this promotes creativity, or does it just focus it? I think the focus makes it engage with reality more, and that this does promote creativity.
This is really how content constraints affect content (not how medium constraints affect content.
Typewritten essays were very constrained in their presentation (especially in the form of the manuscript that the author worked with), including academic papers, such as Einsteins.
PS: maybe I misunderstand, and by "content" you mean the presentation that people create, not the text.
Your observation about 6 types of content is fascinating -- it makes so much sense! The medium is the message, right? So content expands to fill the container it is given.
Given a blank page, people will create anything. You can take three HTML-only pages at random and you'll come up with 3 very different results.
Given a blogging engine, people will start restricting themselves to the form of their engine. That means you get lots of different posts, but very rarely will you see a blog using the Wordpress engine for something really standout and unique.
Given a system that divides posts into 6 types of content, the vast majority of blogs created will all start looking the same. The average tumblelog is self-referential, generates long winding tags of citations upon reblogs, and content tends to be amateur and haphazard. Very rarely is a tumblelog interesting or a source of good original content, and deviations from the norm are even rarer.
I'm in the middle of writing an essay about how medium affects content - it's taken about 5 months so far, so perhaps it's not quite even "in the middle" yet - and the gist of one of the things that I state is that the more defined your form is, the more rigidly people adhere to it. And form is good for some things. For other things, it's better to keep the form as loose as possible and let people riff on their own.