This is new to me, but you can now link your Discover card with your Amazon account and use available cash back bonus to pay for purchases (if over, you'd just pay the overage with your Discover card). While it is good that you can have access to your bonus to use at Amazon (and I would assume not in $20 increments...) it is bad because I'm sure I'm not alone in wishing that Amazon would have just partnered as a retailer and perhaps even offered a little extra for redeeming it as a reward like many other retailers do (eg. $20 gets you $25).
I suppose that some might like this because the list of retailers has had some drop off in the past few years, the only real convenience is having access to your money instantly versus requesting a check, loading it onto a branded Discover card, or redeeming it for a participating retailer's gift card and maximizing the most out of your hard earned cash back reward money. With the dollar-for-dollar conversion, I'll just redeem for other retailer cards that offer a bit more bang for my buck. Still, if you had a lot of money sitting there and found something cool on Amazon that you couldn't easily get at another retailer, it might be worth checking into.