I think there are quite a few differences between a weblog and a discussion board in terms of creating a community and the community's characteristics.

I believe that discussion boards aren't very welcoming to outsiders. The 2 forums I participate in, are very inwardly focussed. It is similar to a group of friends in an American High School. A forum is like the cool kid's table at lunch. There is a history, standard methods of communicating, slang, intimate knowledge and opinions of others that develop in a forum.

The people in a forum write for each other. The posts are often terse, short and without a lot of context.

A weblog is more welcoming. It is written for someone that may just happen to be strolling by. So, it is written for an unknown audience. Weblog posts are usually more formal, well thought out and structured to make a point.

In forums, threads die and are usually deleted or archived after a period of inactivity. lso, as I mentioned above, forums are usually isolated from the rest of the web: closed off islands. The information is not stored in infinitum either, like in weblogs...

Weblogs have permalinks and they link to outside weblogs, sties, etc. They weave a web of conversations across the web, which serves to unite disparate islands like bridges.

In the world of marketing, I see weblogs as a customer acquisition tool, whereas forums are a customer retention tool.