[jedbrown]

Population bias? It sounds like you currently have a sizeable population of anonymous users that evidently have found value in the site because they keep coming back. They are easy to convert now, but may have written the site off had you shown them the overlay on their first visit. Once you exhaust that population, you may get fwer conversions than before and fewer people using the site. How are you distinguishing the anonymous users that leave in disgust and vow never to come back? Are you sure you want to alienate such users up-front? I strongly prefer StackExchange's approach to Quora's.