StackOverflow reputation has turned into a handy currency for people to use in determing expertise. Some employers are using it; some people are using it as a useful proxy measure for inquisitiveness, knowledge, communication, programming ability (hmm... doesn't correlate with being able to answer questions), intelligence (double hmm...), sex appeal (now we are getting silly) and so on. But the raw reputation score isn't that useful if you want to find out how good the person is in a particular language or technology niche. The fact that someone has 5,000 reputation is a nice thing to know if you are trying to decide whether to give them a job as a C# developer - but if that's 5,000 points all from subjective and not-programming-related, it's not quite so much of a ringing endorsement.
Would it be possible then to expose reputation grouped by tags for users? Would it be desirable? Would it be possible to have in the API? This data is already available in the form of the leaderboards on the tag stats page - e.g. c# - could and should it be available on the user profile and potentially elsewhere?
The API brings out some possibilities for using this data elsewhere. Imagine php.net reborn with StackOverflow reputation information from PHP-related questions sitting next to posted code samples in the comments section (some of which is awful, old deprecated code). Same for blogs, issue trackers and the like. Imagine if you are working on a Java library - you log into your bug tracker and sort the newly-reported issues by StackOverflow reputation for the java tag. You'd get a rough idea that the issues reported are likely to be from people who don't need to be pummelled with a clue-by-four and can address them first (alternatively: if they are so smart and have all that reputation, they ought to fix it themselves and send a patch). Ought it be possible for external sites to be able to use this kind of data?