Maybe this was already known, but I missed it. So guilds are account-based according to this:

http://www.youtube.com/watch?v=Cwsc_VknApg&t=5m55s

Not sure how this works, or if this is a good thing or a bad thing. I generally have characters with different themes who would fit into different themed guilds, so at face value this seems like a bad thing - but then this is like a 15 second snippet with, like, no information, so... Is there more info out there?

Yes, guilds are account based, that's why you can join 5 guilds with one character. If they didn't do it account based then we probably wouldn't have this problem where you could join more than one guild with the same character.

So...all your alts are all in the same...guilds? Or you can just join 5 guilds per account, not per character? I am confused.

I share the same confusion with you on this subject my friend.