One well-known failure mode of wikis, infamous back when they were a big thing, is that pages are often born as a conversation, a glorified forum thread, then stay that way. Wiki people see that as a bad thing: those pages, they say, should have been edited into "proper" articles.
That however presumes the conversation itself has no value, and it does. Moreover, like I pointed out elsewhere, it matters whose words they are. People naturally want credit and authorship: to own their words, and for others to own their words, too.
Conversely, all too often I see a forum thread where the original post keeps being updated with resources suggested by other participants. That's when they should have had a wiki as well, it makes me think. Not instead of a forum, but complementing it. What a concept!
Can't have an online community with only one kind of communication tool.
Ironic how I'm writing this on a blog. It could easily be on a forum instead. After all, both are organized around newest-first lists of posts, each followed by comments. But it's just not the same, isn't it?
Wikis, too, for all their supposed atemporality, consider a Recent Changes page absolutely vital. And they're right! It matters when words were written, as you'll hear from anyone frustrated by the lack of a date on a long-sought-after blog post.
But old blog posts all too soon end up buried under an avalanche of newer writing. Tags, categories and archive pages help a little, but not enough. Some people have tried marrying a blog to a wiki, making a so-called bliki, but I'm not convinced. Ease of editing wasn't the issue. Diving to the bottom still is.
Blogs need to end at some point, not unlike books. Wikis and forums are tricky.