The following stats are based on Baymard Institute's 2012 checkout usability database of the top 100 grossing e-commerce sites, with 508 checkout steps reviewed in total.
The average length of a checkout process among the top 100 grossing e-commerce sites - counting from "cart page" to page where the order is placed (most often a "review order" page)
The top 23 sites with online sales of more than $1 billion scored lower than the 77 other sites on average
Newsletter opt-out is the most common (mal)practice. In fact, 32 of the sites automatically signed the customer up to their newsletter with no way of opting out during checkout
Instead of pre-filling fields, these sites make the customer enter the same information multiple times (typically in different steps), thereby adding needless friction to their checkout experience
While this may seem like a relatively advanced feature, the lowest grossing site benchmarked had $148 million in online sales (Internet Retailer 2010) and could be expected to invest in features like this
However, 12% (relative) of these don't allow the customer to override the validator in case the address isn't recognized but the user is absolutely sure the address is correct
These accordion checkouts have almost the same length as traditional checkouts with 5.29 steps on average