I find the favorite and ignored tags a very useful feature, and I use many of the same tags for several different subforums. The problem is that tags are usually not added in bulk, but as I browse on the fly. So adding just a single favorite tag becomes a tedious process of clicking through pages. Keeping track of what tags are saved for what subforums becomes confusing.
The favorite/ignored tags could be split up into two sections: one for the current site, and the other for all sites.
(A related question was split from this one and posted here.)