This is gonna start off sounding off-topic. Bear with me...
Weird Al Yankovic understood something about online culture back in 1999, that seems to be less widely understood since then. Allow me to quote from "All About the Pentiums":
Hey fella, I bet you’re still living in your parents’ cellar,
downloading pictures of Sarah Michelle Gellar and posting "me too" like some brain-dead AOL-er,
I should do the world a favor and cap you like Old Yeller,
you’re just about as useless as Jpegs to Helen Keller.
Wow, Al, that's harsh! What's so horrible about the "me too" post? It adds nothing to the discussion! If a message would be relevant to just one person, email that person directly. This has been a problem since the invention of reply-all, and a mailing list (aka discussion board, aka group) is like a 24/7 reply-all-fest. It requires discipline and cultural understanding on the part of its subscribers.
... overwhelming the old-timers' capacity to acculturate them...
There's that word again, culture. What's culture, anyway? I like Ifte Choudhury's definition: "A culture is a way of life of a group of people--the behaviors, beliefs, values, and symbols that they accept, generally without thinking about them, and that are passed along by communication and imitation from one generation to the next."
Passed along by communication and imitation. Okay, well, a mailing list is all about communication, that part is easy, but there's a breakdown here -- when a cultural value like "email someone off-list instead of posting me-too replies" is an absence of public behavior, it's impossible to imitate. When someone gets it wrong, everyone sees that, and imitates that. But when someone gets it right, there's nothing to see, nothing to imitate. And if the whole thing breaks down, we feel less like a group of people. That's bad.
(I'm gonna handwave past the "generational" notion and say that online culture moves faster than human mortality. This post is getting long already!)
So how do we try to remind and reinforce a cultural norm for the absence of a particular behavior? Maybe posters with McGruff The Email Dog saying "Don't do me-too!"...
Yeah, that'll work.
How about with explicit reminders right in the body of the text? Quoting the OP here: "Reply to me directly" -- that means direct email, off-list. Reply to author. Whatever your mailer calls it! When someone goes out of their way to say that, it means they realize before posting, that their post has the potential to start a flurry of useless traffic, AND that they know that not everyone receiving the message has the cultural background to automatically do the accepted thing, so they're gonna shift into a bold font and make it really clear, right up front. And hey, a dozen people might have done exactly that, but we don't see them.
Oh also, on the "communication" front, there's http://i3detroit.com/wi/index.php?title=Mailing_List
with lots more detail. These guidelines are distilled from decades of observation of what works in online communication and what doesn't, and are intended to keep us from wasting each other's time. A shocking amount of traffic on this list falls into the "me-too" category. Let's fix that.
p.s. I just tried setting a "welcome message" on the Google Groups web interface, which points to the above guidelines. I don't know how many people interact with the groups through that interface, but it might help make some things a bit more clear going forward.