Okay, let's take what you're trying to do aside from technology for a second and ask yourself: what is it you're really trying to accomplish? Subforums are a means to an end, not an end in themselves, so you've probably got some sort of need you want to meet. What is that need? Would installing complex software be the best way of accomplishing it, or could it be achieved in a 'human' way instead?
I've been administering a forum for a few years, and in that time we've switched from the (inordinately complex and woefully insecure) PhpBB2 to PunBB, but even the latter is too 'complex' for my liking. Simple Machines seems to suffer the same sort of featureitis (the 'Community' link on their own site takes me to a forum page so densely packed with information I'd not know where to start).
If you're managing a World of Warcraft guild, first of all; don't try and emulate Blizzard's forums. They have lots of forums on lots of different subjects because of the sheer volume of posts, and because it'd not be manageable any other way. If you've got one person asking a question about (say) Blacksmithing in a week, does that really necessitate having a dedicated forum for Blacksmithing? Does it even necessitate having a dedicated forum for Professions? Take a step back and think what you really need to be able to separate (i.e. what types of post wouldn't work very well if they were mixed together) and then separate down to those few things.
As far as technology goes, I'd recommend something like
Vanilla over PhpBB, PunBB or SimpleMachineForum. Even better would be
Beast, but that doesn't currently support private forums (although I imagine there are 3rd party mods that do), and it's written in Ruby on Rails, which might not be so easy for you to deploy if you don't have prior experience with it.