If the person is speaking as an editor or as an amv "critic" then the join date is my first clue about how to take what they're saying.
Someone who has a really old join date (2000, 2001, 2002) probably remembers things about amvs that are "before my time". If they talk about things like trends and changes in the community, I tend to believe them more than I would if it were someone who joined yesterday. It's snap judgement. When someone comments on one of my vids and says something I don't like, I tend to put more weight on their opinion if it looks like they have more experience than me (from being here longer, and thus seeing more). I still probably won't change anything for them, but I won't argue as much as I would if it were someone who just joined.
Age is a factor, too. It's no coincidence that people who talk (type) like they're little kids tend to have recent join dates. Face it - if a person has been a member here for 6 years, he's probably older than 14. If I'm asking for advice, I'd go to the person with more experience. Experience comes with age more often than not.
On the flip side, when I read something really significant from a brand new member, I'm more impressed than I would be if it were written by someone who joined years ago. Same with videos - when someone can churn out excellence on their (seemingly) first try, it's more impressive. We expect more from those who've had years to practice. And if you've been here a few years, you're more likely to be jealous/ashamed when you're shown up by a "newbie" with no experience under his belt.
It doesn't matter off the forum because - aside from the journals - you probably won't ever talk to each other, or see who's most active, looked up to by his peers, etc. Even when you're downloading vids, if you use the search you probably won't pay attention to the join date on their member profile page. People don't even pay attention to the date the video was made. It's not likely they'll look at the creator's join date.
Join date is like a beard. Do you treat gray-haired men differently from babyskinned little boys? I do. At least until I get to know them personally and find out if they're really as different as they look. You can't judge appearances by looking at avatars, so the join date (and user name for the dbz people) gives you your first impression.
Post count, on the other hand, is pointless. Useless spammers are more likely to have a high post count than anyone else. It's a good thing that page
isn't linked up front anywhere or...yeah, we might have people getting the wrong impression about our most active posters.