When someone makes a thread about mine carts or whatever, the mantra is "this has already been discussed, here are the two or three huge threads that we already had about it, we don't need more threads about it", so I don't see what the point of making topic-specific subforums would be, since they would be places designated for making more threads about things that we don't need more threads about. Do we really need that?
And if subforums break searches, then you can't search for all the things that have been said about mine carts, and you can't find out what's been said by reading the mine carts megathread, because now we have a whole forum full of threads about different aspects of mine carts, which may be redundant because their posters couldn't search either.
I'm not convinced this would actually serve to make things more organized.
I'd rather do things that streamline the process of finding out what suggestions are common. In addition to or instead of a wiki-based index of important threads about common topics, it might be nice to have a "commonly suggested ideas, read before posting" sticky thread, posted by some community-minded volunteer, stickied by Toady, then edited to stay up-to-date by the aforementioned volunteer. Yeah, that kind of indexing will take work, and controversial editorial decisions about what ideas deserve inclusion, but that would make things more organized. Adding more complexity by splitting things up into a maze of subforums is something Toady could do with less work (if possibly no less controversial editorial decisions), but it would not make things more organized on its own.
I'd also see some potential in strictly janitorial moderators, who don't ban people or intervene in disputes or otherwise engage in dramatic assertions of authority that stir up discontent, but just go to redundant threads and paste some kind of "Welcome to the DF forums. Your idea's been posted before and discussed at length, check out these threads: (links taken from common-suggestions sticky list)" boilerplate message, and then lock the thread so that it doesn't keep getting bumped and attracting a crowd.
We have a sticky thread for the cat bug in the bug reports forum and there's no reason it shouldn't work the same way for the most common suggestions. Come to think of it, Gameplay Questions could use the same kind of thing. I've been paying attention to it this week and it seems like every day there is a brand new "I can't get the hang of the game because in every fort I suddenly get a hundred migrants in my second year, what do I do?" thread.
It seems like similar mechanisms could be used to better organize Bug Reports, Suggestions, and Gameplay Questions. Take some community nominations for commonly reported non-bugs like when people fiddle with their raws and get indexing errors that turn all their elves into elephants or give them "cancels make steel: need some crazy nonsense material unrelated to steel" messages, commonly suggested ideas, and FAQs like "how do I stop migrants?", and make some sticky threads.
Incidentally this also highlights my issues with the subforums idea. Does anyone think it would really be better organization to set up a system of "to ask questions about construction, use the construction questions subforum; to ask questions about migrants, use the migration questions subforum; to ask questions about fluids, use the oh shit I flooded my fort subforum" and so on?