How about allowing members to add tags to others' posts/questions?
It could be done without too much risk of vandalism. It could require a certain reputation to be enabled. Or it could allow only addition of tags and not removal. Or it could allow adding tags only from the preexisting tag list (or, getting fancy, the full list from a few days earlier).
There is a huge volume of very nice old posts buried in this site. Without tags, it will be much harder to sieve them out of the mass of regular search results. But without the ability to tag others' posts, much of the hidden treasure will stay hidden.
note to self: with this post, I get to create the "tags" tag.