Our store has been up for about three years, about 4 months with zencart. We have lost about 30 or so orders a week on average (our sales have dropped, and it is particularly less orders for one item specifically) because we now require a login, which may not seem like a lot, but it's enough.
We offer a few small items that people buy when they first start a military or police academy and usually never again. They like our prices, but they also like the anonymity and ease of not having to create an account for a single $7 item.
We get a few calls and e-mail a week asking if they can order without creating an account, and we take the order by phone, which works, but takes more time.
I have seen a "hack" that actually creates an account, but doesn't require an e-mail, which is not useful. We, of course, require an e-mail to send an order acknowledgment and a tracking number if they order express. We used to send a "Sign up Now for these features" text at the bottom of the confirmation e-mail.
I wish I had the time to get a real order with creating account mod working on zen cart, perhaps in May or June during my slow summer season.