What are the salient features of Romanticism?

Revolution that swept Europe and America around 1776 created within artists a new appreciation for imaginative, unrestrained literature and other art forms. The Romantic literature that flowed out of this period embodies several key characteristics:
Imagination: Consider the following lines from John Keats's "La Belle Dame sans Merci":

I met a lady in the meads,
Full beautiful—a faery’s child,
Her hair was long, her foot was light,
And her eyes were wild.

I made a garland for her head,
And bracelets too, and fragrant zone;
She looked at me as she did love,

Keats combines an emotion—love—with imagination. He has become enamored with a fairy woman. This isn't a realistic portrait of love or adoration, but that is key to the Romantic movement. There are endless possibilities, thanks to the imaginative power of literature.
Idealism: Consider the following lines from Lord Byron's "She Walks in Beauty":

She walks in beauty, like the nightOf cloudless climes and starry skies;And all that’s best of dark and brightMeet in her aspect and her eyes;Thus mellowed to that tender lightWhich heaven to gaudy day denies.

In this poem, the narrator presents this impeccable beauty, one that even the brightness of day impairs because it is too "gaudy." At the conclusion of the poem, the reader learns that this woman's innocence and virtue match her immeasurable beauty. She encompasses the ideal characteristics of women at that time.
Emotion: Consider the following lines from William Blake's "A Poison Tree":

I was angry with my friend:I told my wrath, my wrath did end.I was angry with my foe:I told it not, my wrath did grow.

Here we see a cautionary tale regarding harboring anger. The narrator notes that when he told his friend of his discontent, he ended the animosity. However, when he withheld this information from his enemy, the anger grew. Literature of this time period often reflected the emotional spectrum, both positive and negative experiences.
Nature: Consider the following lines from William Wordsworth's "Daffodils":

For oft, when on my couch I lieIn vacant or in pensive mood,They flash upon that inward eyeWhich is the bliss of solitude;And then my heart with pleasure fills,And dances with the daffodils.

In this poem, the narrator conveys that this beautiful memory of a scene of daffodils often lifts his mood when he is feeling rather gloomy. In this poem, the daffodils "dance" in "sparkling waves of glee." This idea of turning to nature for inspiration is key to the Romantic movement.
Simplicity: Consider the following lines from Percy Shelley's "To a Skylark":

Hail to thee, blithe Spirit!Bird thou never wert,That from Heaven, or near it,Pourest thy full heartIn profuse strains of unpremeditated art.

In these lines, we see that the narrator sees beautiful art in the simple call of a skylark. It was inspired by a simple evening walk that Shelley took with his wife, Mary.

In each poem, there are characteristics of the Romantic movement that overlap, but these authors and these excerpts show that the Romantic movement was a shift from the neoclassicism period prior to it, which focused on reason, order, and tradition.

Romanticism was a literary and intellectual movement that lasted from the late eighteenth century through the mid-nineteenth century. Classic examples of Romantic novels are Nathaniel Hawthorne's The Scarlet Letter and Mary Shelley's Frankenstein. Though academics consider Romanticism difficult to define—the movement developed differently in European countries than it did in the US—there are a few key features we can talk about.
The first is important: Romanticism was reactionary. The movement was, at least in part, a response to the Industrial Revolution and the Age of Enlightenment. Rather than focusing on science, logic, or reason, as was the zeitgeist on both sides of the Atlantic, Romantic writers were nostalgic, looking to a simpler past for inspiration. Much as we, as contemporary readers, may look back to the pre-internet era with some sentimentality (remember when we looked things up in an actual encyclopedia instead of Googling everything?), Romantic writers fondly remembered a pre-industrial era.
Which brings us to our second point. Romantic writers expressed emotion and imagination, engaging with aesthetics and the beauty of the natural world. In the poetry and novels of the era, emotion was more important than reason or science. It stands to reason that Romantic writers also rejected some of the structure or rules that had previously governed both novels and poetry, experimenting with freer styles.
All of this could be summarized with a third point: Romanticism is about the individual and his or her thoughts and experiences, not the trends or tendencies of a larger society.