When I first started tinkering with hosting blogs on hoeben.net, I thought it would be a nifty idea to base it on phpBB. One of the sites I visit regularly is myHTPC.com, and I liked their idea of the whole phpBB based 'portal'; everything that happens on the front page comes from somewhere in the bulletin board. Different authors have different (moderation) rights, and the whole thing looked pretty spiffy to me.
Toying around with phpBB I found out that it just wasn't ready to do what I wanted: have a number of differently styled weblogs, and have them aggregated on a main page. Sure, with some hacking and slashing, it could be made to publish rss streams, and have different parts of the system display different themes. However, I found that creating themes was far too complicated, and tweaking the system to my liking was looking hard.