Very simply, I'd like to find out precisely how much reputation I've received from each tag.
As it is, I can see the number of tags that I've answered / asked on my profile page under the "tags" section, but what I can't tell is their relative weighting. If I've answered 10 questions on .NET and gotten 1 upvote each, but I've answered 8 questions on Java and gotten 3 upvotes each, it seems pretty clear that my value to the community (as measured by rep) is higher for Java than it is for .NET, but there is no way of determining that from the profile page.
What I'd like to see is a way to determine just how much reputation I've received from any given tag. Obviously, this would not add up to my total rep, since questions get tagged with multiple tags, but it WOULD give me a very good idea of where my efforts in the community are best received. In the same way, being able to see someone else's reputation based upon the tags they have answered would allow for increasing (or decreasing, for that matter) the confidence one could have in that person's answer.
What is the feasibility of this request based upon the back end implementation? I know that reputation can be recalculated based upon logs (I was the recipient of a negative recalculation recently), so I know the information is in there somewhere...