I do not see how Discover is subprime, but if a criterion is a complaint on consumerist.com, then nearly every CC issuer is subprime.
http://consumerist.com/2008/03/asking-for-lower-apr-gets-juniper-itunes-rewards-visa-card-closed-against-mans-will.html.
http://consumerist.com/2011/08/amex-thinks-youre-your-mother-refuses-to-change-its-mind.html
and many others.