It makes Jon Skeet harder to submit answers to all the questions :) If seriously, as a passive user I would agree with your point, but as an active one I'de like to be a leader and such segmentation would encourage to achieve that. Also specific sites attract proffessionals of the subject.

I can see your point. I just think the same could be achieved with everything together. Questions could be assigned a category. Each category could have it's own associated css style and simple URL to give it a uniqueness. Also user reputation could be calculated on a per-category basis in addition to a site wide basis. You would end up with little kingdoms and little kings, while keeping it all together.

Just an idea.

But with the community process of the different Stacks I have a feeling there are reasons other than technical for the separation.

Small, niche communities are generally easier to manage and yield more valuable content. I think this is why reddit decided to go with subreddits over tags.

Well from a user point of view, there are apparently very different opinions on the topic. I'm personnally very glad to have the separate Ubuntu and ServerFault sites.

From a SEO point of view, having separate domains with high density of topic-related keywords works great, as far as I can tell :)

The reputation system is independent of the tagging. So a big site lets you earn reputation in one category and spend it trolling another. Conversely splitting into too many sites means that reputation in a category that spans sites has to be earned twice which risks deterring experts.