Regarding the one-in-all search. Back in the preview board days we did actually have this. It worked by lumping all searchable data into one table. The problem with this is that MySQL isn't very good at managing large busy tables. Even on a modestly sized community, this search index table would be receive many write / read and delete requests. Generally speaking, MySQL locks the whole table when one process accesses it. You can use InnoDB which has row level locking but then the MySQL full text search doesn't work on InnoDB so we can't use that.
We found in testing that the table would lock frequently causing other processes to queue (effectively piling them up). During peak times, this could build up and cause MySQL to crash. This problem would only get worse the busier and large a forum gets.
We decided to scrap this idea and work on Sphinx integration which is separate software that specialises with search indexes rather than try and force MySQL to do a thing it is not good at.
Bluntly, it would have been a disaster.
Ajax searching, live searching, etc are all grand ideas that are very easy to implement from a technical point of view but they are impractical in the real world.