The folksonomy (what a wonderful word) is the absolute key. I've pointed out to mk several times that if it doesn't scale it's doomed and the primary disadvantage of tags is drift. We used to have a bunch of interesting stuff in #writebetterdammit but then the influx of newbies started populating #writing instead. Grouping tags is the bugbear of any online community. I like the way Pearltrees does it (a number of us were in discussions with them to do a client-side subreddit index) but it isn't without its faults either.
Perhaps upon typing a tag, it pulled out tags with that prefix in a dropdown menu; however in addition, a count of users that followed the tag were visible: ecology (22)
economics (231)
economy (14) Not sure about the numbers. That might be a double-edged sword. It might just be enough to show what tags are out there.