One could argue that the main idea of Gulliver's Travels is that we must always be prepared to challenge our cultural preconceptions. As with anyone, Gulliver harbors many such preconceptions and takes them with him as he ventures out upon his epic voyages. But over the course of his numerous adventures, he finds his worldview challenged by the many strange customs and traditions practiced by the different cultures that he encounters.
For instance, Gulliver is forced to reconsider the rationality of his own society's moral values by the Houyhnhnms, a race of rational equine creatures whose intelligence and civilized behavior stand in stark contrast to the unspeakable ignorance and loutishness of the more human-like Yahoos. Under the Houyhnhnms' close, curious questioning, Gulliver is forced to acknowledge the irrationality of certain facets of English culture, particularly those relating to political life.
Gulliver's preconceptions concerning beauty are also challenged on his travels, this time by the stay on Brobdingnag. Gulliver's small size relative to the race of giants he encounters here allows him to realize that standards of beauty, if not exactly relative, do at least differ from one culture to another. When Gulliver observes people up close and personal, he sees that even those who are beautiful from a distance are really quite ugly when their physical features are magnified.
In addition to those main themes noted in other educators' responses, another main idea is the notion that people are different and that one should not judge people based simply on appearances and should instead make allowance for the many differences between people. For instance, when Gulliver first sets out on his travels, he lands in Lilliput and meets the people of Lilliput: the Lilliputians. Lilliputians are tiny in stature but nevertheless are an industrious group who work together to accomplish tasks that seem beyond their capability—almost like ants in an ant colony.
For instance, despite their miniature size, while Gulliver sleeps after coming ashore at Lilliput, the Lilliputian people are able to bind him to the ground. Essentially, they overpower him by working together. He awakes to discover that his arms and legs were “strongly fastened on each side to the ground,” held down by “slender ligatures.” His encounter with the Lilliputians shows the danger of judging people in advance based on their appearance, and it also shows the benefits of community. While one Lilliputian might have no power against him, as a society they are able to dominate him. Both Gulliver and the Lilliputians judge one another by first appearances are are wrong in their initial assessments.
For instance, the Lilliputians not surprisingly are fearful of Gulliver when they first find. After all, they are no larger than six inches tall, and he towers over them when he stands. Therefore, they tie him up so that he cannot hurt them. However, once they begin a dialogue with him, they come to see him as a friend initially and welcome him into their society until they ultimately turn against him. This shows that people should not judge others based on initial appearances.
Moreover, when Gulliver meets the giant-sized Brobdingnagians, they view him as a freak even though it is Gulliver who is normal-sized according to his/our world view. Similarly, when Gulliver meets the Houyhnhnms, they view creatures who are like Gulliver as savages based primarily on appearances.
The main idea behind Gulliver's Travels is to persuade Britons to reform their own society. Swift uses his gullible narrator, appropriately named Gulliver, to show through his eyes a number of comically cruel and absurd fictional cultures. Not surprisingly, the beliefs and practices of these societies, if exaggerated, are similar to common beliefs and practices in Britain. Swift point is to satirize the cruelty and irrationality in the Great Britain of his day.
For example, the Lilliputians Gulliver meets are tiny and pretty, but petty and cruel, while the Brobdingnagians are huge and ugly to Gulliver, but kind and humane. Here Swift satirizes the tendency to assign positive moral virtues to people who are outwardly beautiful and condemn those who are ugly rather than seeing through to their souls.
Gulliver finds himself witnessing all sorts of absurdities in his travels. At the Grand Academy in Lagado, he sees money and resources wasted on pointless and sometimes cruel experiments, a satire of the British Royal Academy. He later encounters the Houyhnhnms, intelligent talking horses. They are far more sensible and humane than their human counterparts, the Yahoos—or for that matter, Gulliver's fellow countrymen.
By getting people to laugh at the evils and absurdities of the peoples Gulliver meets on his travels, he hopes also to get readers to dwell on their own evils and their own unquestioning acceptance of cruel customs—and perhaps change their ways.
It is difficult to narrow down the scope of this satire to just one idea, but, broadly speaking, we could say that the major theme of Gulliver's Travels is that human beings are inherently flawed. Though we are capable of some good, often our xenophobia, pride, and lack of humanity in many situations leads us to behave selfishly and without compassion for others. The purpose of satire, however, is not simply to point out someone's flaws; it is to point out someone's flaws in such a way that it will open their eyes to their mistakes and prompt them to change their behavior. Therefore, Swift doesn't simply want to point out our stupidity or our ignorance, he wants to compel us to reevaluate the way we think about many things: religion, war, logic and emotion, immortality, and the list goes on and on.