I would argue that the central idea of John Keat's poem “La Belle Dame Sans Merci” is the difference between appearance and reality and the consequences of mixing them up.
The knight meets a fair lady, “a faery's child,” who is beautiful, and he falls deeply in love with her. But there is much more to this lady than the knight realizes. Everything seems wonderful to the knight as he pursues his love. He makes garlands for the lady's head as well as bracelets and a belt, and she accepts him with apparent love. He places her on his horse and listens to her song. He eats the food she gives him and hears her proclaim her true love. He goes with her to the “Elfin grot” and kisses her as she weeps. Then he sleeps and dreams a horrible dream and wakes up into reality.
Now all appearances have fled, and the knight finds himself on a cold hill, caught up in the lady's spell with other kings and princes and warriors. Like them, he, too, is pale and lonely, now facing the reality of his love for the lady.
The lady is not at all what she appears to be, and she has given the knight some clues that he might have used to discern the reality beneath the appearance. Her eyes are unusually “wild.” She moans as she looks at him. He takes the moan for love, but it was not. Rather, it may indicate her desire to bewitch him. Her faery's song seems innocent, but it probably is winding a spell around the knight. The language in which she declares her love for the knight is “strange.” She weeps and sighs, perhaps in regret that she must turn this knight into a pale shadow of himself, yet she does it anyway. Indeed, this lady is not at all what she seems. Beneath her beauty and charm, she is sinister in reality, ready to capture the knight in her spells and entrap him on the “cold hill side” to which she has led many others.
The knight has failed to discern this reality beneath the lady's fair appearance, and the consequences of his error in judgment—his mistaking appearance for reality—are dire indeed.
One of the main ideas in "La Belle Dame sans Merci" is that a love constructed between those from different worlds is sometimes doomed from the start.
In this poem, the speaker meets a knight who has become infatuated with a lady who is a "faery's child." While the knight represents a world of order, discipline, and service, the fairy lady represents a world of mysticism, free-spirited passions, and endless possibilities. While the knight is immediately smitten with the fairy lady's beauty, he also recalls that "her eyes were wild," reminding us of her untamable spirit. It is the lady who professes her love for the knight:
And sure in language strange she said—‘I love thee true’.
The knight makes...
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no such profession; although it can be inferred that he engages in sexual activities with the fairy lady, most notably in the lines about the "pacing steed," the knight never returns her sentiments of love. The knight's sense of honor is thus called into question. Perhaps it is this lack of true emotional connection that causes the fairy lady to weep and sigh once she returns to her "Elfin grot."
In this poem, the wild and mystical world of a fairy meets the disciplined and structured world of the knight. Although he certainly admires her beauty, her proclamations of love are unanswered. The two lovers are from different worlds and thus have differing goals for this intimate encounter. Their lack of ability to truly connect demonstrates the difficulty in attempting to construct a relationship between those from two different backgrounds.
It is important to note that poems are not essays. They do not require a single controlling idea or any ideas per se. While they can be grounded in abstraction, most poems are organized around narratives, perceptions or emotions rather than ideas.
"La Belle Dame sans Merci" is a narrative poem telling the story of a traveler encountering a pale, sickly, melancholy knight. When the traveler asks what the knight is doing wandering around in the remote countryside, the knight tells a story of having been seduced by a female fairy or elf. After a ride and extended foreplay, they have a (discretely worded) sexual encounter and while the knight is asleep, the woman leaves. Since then the knight has wandered mournfully around the area of their meeting.
One can read the poem romantically, seeing the knight as a victim, seduced by the beauty of the fairy. The central idea can then be seen as focusing on the longing for beauty being fatal and destructive but still leading to moments of ineffable joy for which the romantic knight or artist sacrifices everything.
One could also look at the poem more realistically. First, the knight blaming the woman for his unhappiness and having him "in her thrall" can be seen as silly. He willingly slept with her. She left. Rather than indulging in melodramatic moping, one could argue that he should get back to his knightly duties. Thus we could also argue that another central idea is that romantic delusions interfere with getting on with one's daily life and duties.
"The Belle Dame sans Merci" means, in French, the beautiful lady without mercy (pity). The central idea of the poem is that beauty and our own illusions about it can deceive us.
The knight in the poem comes across a woman, saying she was "full beautiful--a faery's child." He thinks she loves him, but this is his subjective interpretation of events. She looks at him as if "she did love / and made sweet moan." She also, "in language strange" says she loved him, or so the knight wants to believe.
In fact, beguiled by her beauty, the garlands she makes for him and the honey and manna she gives him, the knight misinterprets the beautiful woman's intentions. She is not in love with him, but has lured him into her trap, so that she can hold him in "thrall" or captivity, like the other ghostly knights he sees.
Keats says here that beauty is a trap that can hold us in thrall.