These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternate Character Interpretation: Morgause and her Orkney Clan consider Merlyn to be a "wicked necromancer" for the part he played in Cornwall's death and Uther's subsequent marriage to Igraine.
Magnum Opus: Generally considered T.H. White's masterpiece, and certainly his most popular book.
Romantic Plot Tumor: The relationship between Lancelot and Guinevere in The Ill-Made Knight can annoy some readers.
Tear Jerker: One of, if not the greatest Tear Jerkers of all literature. The ending, where it all goes down, and it's all so inevitable, and the last page, never fails to bring tears.
Gareth's death. The tears came when Gawaine and Lancelot began talking about what he was like, and poor Lancelot not only loved Gareth, but didn't even remember killing him.
Tough Act to Follow: Although White wrote many books, none of them were as successful or as popular as The Once and Future King, and with the exception of five: the original edition of The Sword in the Stone, Literature:The Book of Merlyn, The Goshawk, Mistress Masham's Repose, and White's English translation of a medevial Bestiary written in Latin, his other works are largely out of print and mostly forgotten.
Values Dissonance: In his discussion with Arthur and Kay regarding just war, Merlyn provides a defense of imperialism, stating that it is wrong for people to revolt against a conqueror, even if the conqueror is oppressing them. Especially when juxtaposed with his opposition to aggressive war, this defense of empire would not necessarily sit well with modern readers.
The Woobie: Lancelot. He's fundamentally incapable of being comfortable in his own skin, to the point where he spends his entire childhood in the pursuit of becoming the ideal knight, at the cost of such basic skills as tree climbing. He wants nothing more than to be holy enough to perform miracles, but when he actually succeeds, the girl he performed the miracle on rapes him (which is awful enough, but it also means he's no longer a virgin and thus no longer able to do miracles, at least in his mind) and then later traps him in a miserable marriage, which complicates his already complicated relationship with Guenever. The only thing he really has — his status as the greatest knight in the world — is taken from him by his own son, who is basically everything Lancelot wants to be. He's just a heartbreaking character generally.