i've run sites with 500K users and no problems... the main issue is having mysql tuned properly... (mysqltuner.pl for the win, Alex!)
wider (ie: lots of fields) tables isn't an issue, if you have your sql engine done right..
that's what indices are for and believe me, with caching and some proper tuning, it's all good..
the only real area where i notice a dire need for improvement is the post indexing and specifically for searching.. we have an ascending index (POST_TIME), but we are 95% of the time returning reverse sorted results!! why not have a parallel index that is by definition DESCending.. (ie: 2099-Now in unix time)
another slightly bothersome table(s) is the Private messaging system. we don't really handle large numbers of PM's well right now..
all of the above runs fine on a dual core Extreme or quad core with proper memory quite well..
of COURSE, i'd never expect to run a large forum on shared hosting, but who would?
