I'm going to second this and add to it a little. I'd like to see a very strong focus on improving performance for the software and more of a focus on helping people optimize their servers. I've been lurking around on vbulletin's site and it seems there are more than a few sites over there that hit 500-800 users online (some at peak others pretty continuous) and it does appear they are doing this on one box, many running other sites on the servers as well. I realize they don't have the threaded option, but frankly with w3t it seems like if you have more than 100 users online the suggestion is to buy a second server. Honestly, I don't think that's an acceptable answer (please excuse my frankness here).
We have a pretty beefy box and it's the forums that kill us, no question. We recently installed a second mysql binary specifically for the forums and when we get any sort of load on the forums and watch the CPU usage jump up to 80%+ in a heartbeat for that mysql daemon. It doesn't take much to get it there. And the number of queries per second is just crazy.
I'd really like to see you spend monster time (or hire a mysql guru) to really look at the structure of the database and tables, the relationships between everything and all the queries the system produces to really trick out this bad boy. Right now it's generating way way way too many queries, especially in the threaded areas. And what I see regulary in the slow query log are really just three or four of the queries that come in over and over again. And have options in the config and setup for things like multiple tables for forum posts using categories to group them in tables where they will improve performance by splitting up things like that. I think a goal of achieving something like 500 (recurring) to 750 (peak) users online on a single server, of course with certain parameters set (ie intel based hardware platform, o/s, mysql binary version, table types, specific my.cnf options, etc..) that can be recreated by your customers is not an unrealistic one.
Just my $0.02
