What about having alternate paths: say 4 hours and 250+ words posted, or 10 hours and a single post? As a long-time lurker who's got some (hopefully!) interesting things to say, the current scheme encourages newbies to come as close to spam as possible to get their 5 posts, instead of encouraging them to post actual content.
Or captcha the first 5 or so posts to prevent bots/discourage spammers?