edit button: probably the #1 feature that's been requested.
forum search will be a high priority
I like the idea of having an archive you can search through, but only moving sticky threads to the new forum. We might need some way of letting users petition that their threads be copied over from the old system to the new system, though, especially those pertaining to creative writing, etc.
For that matter, we could probably just scan the existing forums and say "anything with more than __ replies to __views gets moved to the new system."
The database we've been using for building part of AG3 has some really nice full text search (FTS) capabilities, which will find English phrases extremely quickly. Unfortunately, it won't find non-English words very well, or things like strings of punctuation symbols if you're trying to find some old thread with ASCII art in it, etc.
Most of the forums I've looked at have their own way of breaking down a forum message (original topic message and every reply) into 'tokens' in order to build efficient search queries. Many forums try to develop their own algorithms of how to best find content. My opinion is that if our forums are English-only anyway, we might as well just let the database handle the workload... that's what it's there for.
One algorithm I wrote using some of the FTS capabilities allows us to use the new search feature on AG3 to pull up a list of about 24,000 usernames (of 1.5 million usernames in the system) to search for the word 'game', plus sort them in a ranked order and alphabetized, in about 90 milliseconds. Part of this algorithm was the database doing its thing, and part of it was me writing some extra algorithms on top of what the database is doing. We'll continue to look for ways to streamline forum searches as we look at other forum engines.