I think there should be a small traffic requirement like 10+ visitors a day, but if there was a traffic limit people might use unethical ways like "spam, autosurf, and Blackhat SEO(invisible text, link farms, etc), and bothering friends, graffiti and stickers of website domains, offline junkmail like printed flyers in everyones real mailbox and family to visit their site daily" to quickly get traffic to get accepted int adsense.
But, I think a original content limit should be more suitable like a minimum of 5 pages of orginal content since from my experience most of the content which makes money on my sites were orginal content written by me which were optimized for a specific audience like "Computer Security Gurus" or "Computer Security Newbies" or a keyword pharse like "windows 7 computer security".
