Far too much complexity for the problems you propose to solve.
I went cross-eyed trying to read the whole post, but I was simply not convinced by the "problems" you posit with the current system: those may call for a better means of telling the system what we are and are not interested in. They don't call for a complete rewrite of the post sorting mechanism.
Remember that we're going to have to explain all this to the users who
- want to search
- want to set their interested and ignored subjects
- obtain the ability to create tags
and that if they don't get it theirthey're going to be here on meta asking about it. Over and over again.
There may be issues surrounding tag approval. Stack Overflow currently has more than 500 10k+ users. Can any one approve a new tag? What if other high rep users disagree? Are we going to end up with a voting system?