It was because of the amount of activity it got.. with all topics on one page it can get abit overwhelming.. though then that's where the categories come into play so personally I don't really see the big problem.
BB did make a mistake in my opinion, but then I've been used to this kind of forum for the last 4 years dating back to o8, and I'd wager o8 was a damn sight busier than Binary Bonsai
It's not for everyone, but they were using the 0.9.3 beta, big mistake
Sprockett I'm pretty sure that Mark has mentioned this will be an option in 1.0, that you can have categories as the default front page. More than a few people have asked for that.
I just started using vanilla for our new forums, and so far, it seems people are not use to the discussion page. So we hot linked to the categories page, to make that the front page for first time visitors.
But most of the forums, except for the first one are only a week old. So I will keep an eye on them.
The major problem right now, is there is now email notification of a new post for the admin or for people posting. So people know when something new is happening without having to keep checking the site.
I agree that a third kind of start page - recent posts seperated by category - is the way forward. For massive forums it could even have a live refresh.
FLC, remember dreamless though? they were using it but it was so stripped down that it was somewhat of an actual joy to use. I think it's all really in terms of how you slice it down to display how you and your users like to see it.