I know that you've already made up your mind @jesta, but I'll give you my opinion regardless.

Now that I've been using chainBB for a bit I feel like 10 steem to register a forum might be too much. I've about 4 forums that I'd like to register. If it were 1 steem, I would've done so already, but 10 steem each is way too steep for me. Once you've a way to ignore forums (remove them from the mainpage, hide them or collapse them) the more forums the better. I don't think spammy forums will be a problem then, since you won't even see them once you hide/collapse them.

I'm sure there are people using chainBB that want to post something on a forum that does not yet exist and don't feel like creating the forum themselves. So what do they do? They don't create said post.

I think you should incentivize the creation of new forums, and 10 steem is not really doing that, in my opinion.