I'd make it so there is the lowest rank at 0 posts, and by then you can only start a thread in the HELLo section and do nothing else. Then, at 1 post or more, you get free access to everywhere but Goodbye thread starting, which requires Full Member.
...so people are forced to do introducing. Also, a whole section, preferably at the very top for being noticable, would be a better solution, as it's more exposed than one thread and wider and a pageful of threads makes your thread more visible than a pageful of posts.
The only problem with a requirement is what do you post? What if someone would rather not start a post like this?
I understand that it could just be a "If you don't like it, leave", but the section for 'Hellos' would possibly get so flooded by new threads (and replies) that it would fill the unread post section (which I use religiously). All this, when a lot of what you're gonna get is "Hey everyone, my name is GFunkMoney and I like this game" or "I guess I have to post here before I do anything else, so hi".
Sure, some people like to post hellos and put their background, how much they like the game, what else they like to do, whatever. But some people don't. Some people don't care about hellos and goodbyes, they just don't get as involved in all of this as other people.
Maybe instead of making it a requirement to post, you could do some kind of reward...say, after you post a hello, you can use a smaller version of the available avatars? That way, once you've been on long enough to get a stock Avatar, there's no reason to post a hello (getting rid of the problem of older members just wanting the reward), but it gives some sort of difference between people who take the time to introduce themselves and those who just start responding. Granted, the Avatar idea is just an idea, changeable to whatever else works.