Following up on a request from Elonka, I did an analysis using the category web structure to crudely estimate how Wikipedia content is distributed.
The result I got was that Wikipedia "is":
9.6% - People 28.0% - Science 10.5% - Culture 16.0% - Geography 6.3% - History 0.8% - Religion 5.5% - Philosophy 1.8% - Mathematics 14.3% - Nature 6.0% - Technology 1.4% - Fiction
The basic principle I used was that each of the categories listed above corresponds to pure X-ness, and that each child category inherits their flavors as the average of their parent categories.[Footnote-1] Each article then has flavors based on the average of the categories they are in, and the totals come from averaging over all articles. This approach has some interesting consequences, the Category:Scientists becomes a mix of People-ness and Science-ness. Category:American scientists would then blend People-ness, Science-ness and Geography-ness, etc. So even though 20-30% of Wikipedia articles are biographies, most are blends of People-ness and whatever the person is known for with the end result that this crude measure only associates 10% of Wikipedia content with People-ness.
Whether this is desirable or not is of course subjective. Consider, how would you count Scientists if asked what fraction of the encyclopedia is about Science vs. Biographies? In some sense the question isn't even sensible since it is not really an either/or proposition and some articles are about both Science AND People. The same problem exists for essentially any set for categories. The approach I used counts these problem cases a little towards each relevant category, but other solutions are of course possible.
On top of this, there is the problem that the category tree... er, web... sucks. There are many places that the categories meander sideways, like Water sports->Sailing->Winds->Wind power, so the descendent grand-children have very little to do with the grandparents. If you want a challenge, find the path that leads Science to Religion and back to Science (yes such a path exists purely through category children). In fact, each of Wikipedia's "Category:Main topic classifications" categories share nearly all the same children just at different depths of organization.
Oh, and then there is the problem that the category structure doesn't necessarily make sense. For example, Natural science and Applied Science are both "Main topics" but their obvious parent, Science, is not.
So anyway, the category web sucks and the idea of breaking Wikipedia content into discrete categories is somewhat nonsensical, but if one wants to try, it might look something like the list I gave above. Lots of Science and Nature (which in practice means all those stubs of living things and astronomical objects). Many places, citites, states and Rambot fodder. Substantial amounts of people, culture, and history (which overlap in a variety of ways), and modest amounts of other things. The fiction number was lower than I expected, but that may be because it was diluted against the entertainment side of Culture. I also suspect that the Science number is bit stacked because virtually everything is somebody's science (be it social science, policital science, military science, etc.)
Whether these results are useful (or even interesting) is a fair question, and I don't know. Aside from subjectively deciding on some starting set of categories, it is an "objective" measure. However one might well get more meaningful results by subjectively sorting a few thousand random articles. I could also repeat this experiment with a different set of basis categories if people have suggestions.
Anyway, I hope this helps Elonka's curiosity and is interesting to at least someone (if only because it makes you think about the logical problems associated with categorization).
-Robert Rohde
[1] The formal description of this form of flow modeling involves solving a set of 200,000+ simultaneous linear vector equations, one for each category. Glory be unto Matlab.