1. The foreshadowed and impending massive fall from grace for Heather Dunbar. She started off as a holier-than-thou principled character, and she is quickly descending to Frank & Claire's level, and you know Frank will win that game.
2. Franks massive backstabbing of Jackie Sharpe, I nearly fell out of my chair during the debate, that was an amazing Frank Underwood moment, and the acting was so good you could see Jackie's heart break on camera!
3. The Underwoods fallibility this season. Claire was clearly out of her depth as UN Ambassador. They were both easily manipulated by the Russians. Claire was incompetent and had nothing left, resulting in a semi breakdown for her. Her character fell a long way this season. Frank made a good point about their being "one chair" in the Oval office, and if Claire wasn't happy about it, she should have mentioned it a long time ago.
4. "America Works" is clearly an insanely unworkable scheme, and yet as a political weapon it serves Frank Underwood perfectly. I love how his character is so shrewd in some areas (political manouvering), and so awful in others (domestic and foreign policy). AW turns out to be a political tool to get him re-elected, and very few people on the show seem to point out to Frank that it is in principle, ludicrous. (The one guy who did got fired!)