I'll be the first to agree to some extent that the posting quality has gone downhill somewhat as the userbase has expanded. I suspect that stems largely from getting more outspoken people who tend to dominate the conversations via posting more often (the internet equivalent of talking more loudly, I imagine) and some of the quieter but insightful posters (I like to think I am one) being drowned out or finding it too much of an effort to put their point across.
I'm not entirely convinced that restricting topics will stem that too much. Some people have their agendas and they want to push them. Some people will always post compulsively. Threads will be derailed (which I remember as being something of a point of pride a couple of years ago, when done appropriately) for better or for worse.
I'm also not sure that the thread creation by proxy route is the best one. I have experienced on other forums a degree of posting right restriction, such as having to make 50 posts or whatever is appropriate before getting the right to start your own thread. This usually gives new users a time to get a feel for the forum before wading in with their pet grievance thread. In addition, this is often backed up by mods exercising the power to take away user's ability to create threads if they abuse it. This can be for a period of a week, a month or more; or in some cases, permanently.
I'm inclined towards that as a preferable system, as I think it allows for a more open conversation but with real consequences if you're being a muppet. I at least prefer this to a vaguely paternalistic system which may shield people from other users excesses of fervor or indiscretions of judgment, but only really provides topics of discussion which are deemed 'appropriate'.
I imagine that this will depend largely on the size of the 'thread creator' list. Although, I question this too. Too small a list and we have limited discourse, too large and why bother with the process in the first place. I suspect getting the balance on this will prove very difficult to achieve overall.
But when it comes down to it, as long as I have a Rebels thread to natter on in, I'm sure I'll survive.