I second (figuratively, since many have mentioned it) the notion that too many categories is bad for participation. I think we have some categories that overlap considerably.
'Ask a Mustachian' for example - most of those threads could easily find a home under DIY, real estate, investing, etc.
We have other topics that have strayed from their original intention. 'Share your badassity' has become 'share your milestone', for the most part. Again, nothing wrong with that, but it is a bit confusing given what the topic description says.
Another comment - I think the 'Introduce yourself' thread has outlived its usefulness. It made sense with 50 participants, it makes no sense today. It is somewhat awkward to comment on people's introductions as it works today. I often find myself wanting to comment on an intro post but I don't - especially when you want to comment on someone who introduced himself a few posts back. It feels a bit rude commenting on that post but not the more recent ones. I'm not the only one who feels this way, since most intro posts go uncommented upon. A better method would be encouraging new 'Hi, I am...' threads for newbees, like early-retirement.org does. I think these would get comments from posters with something in common with the newbee, and make him or her feel more welcome. Also, that wouldn't need a new category, just lock the current thread and edit the first and/or last post to say 'please introduce yourself with a fresh topic rather than a post here'. Or something like that.
Anyway, those are my 2c.