On Academia.SE, we often have questions which are best answered by our site, but which are highly relevant to other sites such as math, biology, physics, cogsci, and occasionally even tex. Most of these questions are a poor fit on their main site, but as most subject matter experts are there, there's a far better chance of them getting a good answer at that site. Most posters try to "solve" this problem by cross-posting, which has all of the negatives and none of the positives; it's closed on the subject-specific site as off-topic, and ignored on the broader due to low expertise.
The current solution is to simply encourage users to remain active on multiple sites. The problem is that there's no easy* way to view questions you're interested in across multiple sites.
That being said, it would be useful if there was a view which would be functionally identical to the "interesting" tab, except that it would cover the entire network, and not just a single site. It would highlight recent questions from tags you frequent (determined automatically) on each site at which you have registered, and provide a useful dashboard for questions that are interesting to you across the whole network.
* Yes, users can create a custom filtered query on the main stackexchange site. This is not "easy", as it requires knowledge that the functionality exists and the willingness to write and maintain a query with all tags you're interested in across X sites.