Ho Yay: The letter that Bill Haydon wrote about Jim Prideaux to his Circus talent-spotter.
In the second book, Luke and Jerry have such a close relationship that they're Mistaken for Gay.
It Was His Sled: It's quite difficult, especially if you read any later Le Carré novels first, not to know the identity of the Mole in Tinker, Tailor.... However, it doesn't really matter as the identity of the Mole is not really a twist and the real meat of the story is Smiley's investigation and how the Mole was being used.
Magnificent Bastard: Karla. He got started as a spymaster running a network in the Spanish Civil War, during which he would have had to be very young. He is Crazy-Prepared, recruiting agents decades in advance of their Cold War use (such as his Japanese asset). During World War II he worked behind German lines where he played Nazi agents back against their own side, and at one point tricked the Germans into shelling their own positions. At the start of the Cold War, he returned to Moscow knowing he had been chosen to be purged, and instead managed to have his denounced executed and himself reinstated. By the late 1960s he apparently is running the British intelligence service, and it takes an outsider to catch him.
The Scrappy: It's very hard to find a fan who likes Ann Smiley.