Provide per-node-type comment threading options.

Use case:
On your blog posts, you might want the comments threaded and in chronological order, with the oldest first. That way, you can read the responses in the order that they happened, and get the full context of the conversation.

With forums, on the other hand, people get totally confused by threading, because they're used to things like PHPBB... they mostly only care about "What was the latest activity?" Therefore, you might want the comments flat, in reverse chronological order.