Aside from Tim's excellent post on the expectation of privacy, there are a host of other reasons.
This that are fundamentally public are information is anything which you are giving to the site. This is the information in your profile, and any of your posts/comments/answers.
Things which are fundamentally private are information relating to preferences, or how you use the site. That is, it is really no one else' business which questions I choose to view. It is not their business whether I get notifications or not. It is not their business how often I visit the site. And lastly, it is not their business which tags I choose to mark as "interesting".
The internet is currently all aflutter with privacy kerfuffles. Gmail's taking a huge hit for what they are doing with individual user search data (Oh, it's also private what my "search terms" are on SO), Facebook is taking huge flak for what they do with individual user data. It has become very clear that people want to be able to "publicize" information rather than be forced to "depublicize" it.
Which leads to the conclusion of this, which is: Why do you want it public? All the possible reasons fall into two categories. You want to know it or you want to act on it.
Satisfy your curiosity This is a terrible reason to make anything public, because many (if not most) people have curiosity about things that they have no business knowing. Satisfying curiosity is simply not a valid reason for making anything public.
Change your behaviour This would or could have a terrible outcome. I've mentioned it before (especially relating to these issues) StackOverflow is about asking questions. The last thing we want is people to "game" the system. This includes things like "Explain the fraud-detection algorithm", "How can I tell who's online", and your request "Which tags are most likely to be ignored/interested". The reason this is terrible? It would be a very STRONG incentive for people to start mis-tagging their posts because they know that more people are interested in [SharePoint] than [CommunityServer]. So you tag it falsly, and thereby generate more interest. The other half of this is people purposefully leaving off relevant tags because they are on many people's "ignore" list. For instance, there may be many people ignoring [r-language], but that doesn't mean that people shouldn't be tagging their questions with it.
Now, if you're saying, "Obviously, people wouldn't do that, it would be bad if they changed their tagging practices just because of this information" then you fall back to reason (1): That you only want to know because you're curious. Which, as I mentioned, is invalid.
EDIT: I also elaborated here.