Anon,
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link for a side by side comparison, complete with acronyms.
As for predictions about what customers will "hate" the effects of Check 21, my feeling is that banks currently sending image statements will get little feedback. However, those that still include cancelled checks with their statements will get open resistance to the substitute check. Like the customer in the original post, they will feel like they should have had some right to say, "No."
Banks may be able to mitigate or eliminate that resistance through advance communication with the customer, but they will have to do more than just send some legally required disclosures.