(1) Lost US Airways lounge access from the Amex platinum (merger).
(2) Lost Continental as a transfer partner for MR (merger).
(3) Lost Continental lounge access from the Amex platinum (merger).
(4) Lost Delta lounge guest access.
(5) Saw serious devaluations in Aeroplan for MR.
(6) Saw serious devaluations in Delta for MR (no more 50% transfer bonuses).
(7) Saw serious devaluations in BA for MR (transfer 1:1 with 50% bonuses to 1:0.8 with occasional bonuses).
(8) No more points lending (you used to be able to transfer more than you had if you earned them back within a year, otherwise they'd bill you).
(9) Lost JetBlue co-brand card.
(10) Lost Costco co-brand card.
(11) Saw Plenti flop, and I haven't really heard much of Bluebird.
A few years ago, you needed 120,000 Amex points for a first-class round-the-world flight with no fuel surcharges and 2 stopovers in addition to your destination. That price has pretty much doubled in points and gone up 100x in cash. It's just not as rewarding as it used to be.
On the other hand, Chase jumped in, offered lower-fee cards ($95 for Preferred vs $175 for Amex PRG) with more valuable transfer partners -- especially before the United devaluation -- and large signup bonuses. They were friendlier, less stodgy and offered a better value proposition overall in 2011 with the Sapphire Preferred. Then they upped the stakes with the Reserve.
Amex should be worried.