Post counts: a metric for how long they have been here, also an attempt to let new members know if this person knows what they are talking about. Yes, post count is a poor metric for level of knowledge, but measuring level of knowledge is not an easy problem(see controversy over IQ tests). Displaying upvotes could be better, but see reddit where people post the same dumb jokes to farm karma, or steal jokes from other subs.
Signatures: People like to express themselves. I'm not a fan of this one either, but hey. It can be useful when you see someone say something intelligent/helpful/etc, and their signature has a list of threads they started. You might find those threads filled with useful info.
Badges: This encourages desirable behavior. It can also help identify those who have a high level of knowledge.
I would add avatars: Have you every heard someone say they can remember faces, but not names? This is an attempt to help members identify each other.
Edit: post metadata: seeing the number of upvotes on a post gives you an idea on whether it is true or not.
This itself causes some issues (like with spammers joining simply for the 'SEO' benefits), but it's enough of a pattern that changing it will often kill a forum dead, especially in the webmaster/tech/marketing field.
Other things that encourage members (but that are often seen as clutter) include reputation systems (like the green bars vBulletin had), forum cash (points given for posts and other actions), fictional shop items (purchased with said forum cash), etc. Again, a lot of people on these forums rather like said systems and often find them useful, even if to an outsider that might be seen as overwhelming.