Darn, ran out of time to edit my comment above!

I was going to mention that lots of very specific plugin-like functionality should not be included in the theme. I've mentioned Captcha's a few times recently as some people have been complaining that bbPress does not provide them by default, but personally I see that as very much plugin material, not suitable for a theme.

However, if some functionalities such as forum signatures are not included in the default bbPress installation then I wouldn't mind that being included in the theme. It's a little weird, but it is 'sort of' theme related in that it is something which is displayed on almost every page of the forum.

When we were discussing it behind closed doors, we decided that adding certain 'standard forum functionalities' would be a good idea to make it as easy as possible for someone to get their forum up and running. Personally I'm not a big fan of having to install a whole suite of plugins on EVERY site I launch. With WordPress I rarely need to install any plugins, it has everything I require straight out of the box and many themes (particularly the 'theme frameworks' and commercial themes) include a certain amount of plugin-like functionality in them too which reduces the need for plugins.

Also, there is a fine line between a plugin and a theme in both bbPress and WordPress. A theme is really just a specific type of plugin.