@maos, @ceedubsnz, @ulfh and @phlai really have it covered!
One thing I didn’t see mentioned explicitly, but which is triggered by Mike’s persona’s, is mobile testing! So let’s take everything on his or Magda’s list, and make sure we test it out not only on multiple browsers, but on some handheld devices as well.
Also, since I work in payments, I want to mention that @maos has some really good points about payment methods. We usually test to make sure all four major card brands are working (Visa, AMEX, Mastercard, Discover) as well as debit (though that doesn’t typically apply to online purchases) and gift cards, which might.