[joeldrapper]

Is there a plugin, or even a built in feature that I haven’t noticed that will allow users to subscribe to topics that they are interested in by entering keywords, and/or choosing categories?

So that then they would be able to see all the relevent posts listed in a special page for them somehow?

[chrishajer]

I think the closest you get to that is tags. There is an RSS feed for each tag, but the concept of categories or keywords does not exist in bbPress AFAIK.