Recently when moving into a brand new technology I've found part of my getting up to speed routine is now browsing through related questions and answers on SO. I still go through the usual steps like reading documentation, prototyping, reading blogs etc, but seeing a list of common things people are trying to do with a technology and the problems they have had has proved really useful.
One thing I have noticed, however, is that this has been far more useful for smaller tags. I've had to get up to speed on both NSIS and WinApi in recent months and the few hundred NSIS questions to browse over were very managable, while the tens of thousands of WinApi questions made it almost impossible to extract value (not impossible, but very hard).
Would there be value in introducing a new categorisation system, allowing users to browse topics within specific tags? Perhaps something where users with a certain rep and the badge for a tag can create topics and put questions into those topics, the list of topics then being visible for a tag.
I know that tags themselves are meant to address this issue, and can see one alternative being allowing better cross referencing of tags, to see for example all the tags that exist along with WinApi.
This could well be a niche odditity requirement, but I do feel that there is a huge wealth of information gathered on StackOverflow and something like this would allow more access to useful questions, letting people get more of a view of parts of a topic they didn't even know they didn't know about.