Lists will vary slightly in how they name the following features of Romantic literature, but those below represent the gist of what the Romantic poets were seeking to write and what makes it different from most poetry that came before. Because Romanticism was such a powerful force in poetry, it often seems today to simply be what poetry is, but it represents a profound break with the past.
An emphasis on nature: If there is one feature that far and away typifies Romantic poetry, it is taking nature as its subject. The Romantic poets loved nature as a expression of the divine, as a sublime force that would bring us close to the Godhead, and as an emotional solace. Nature moves the emotions, lifts the soul to a higher level, is the antidote to civilization's corruption, and is endlessly a subject that the Romantics explored. In a shift from the past, nature is almost never seen as an enemy or dark force, as it is, say, in Beowulf but as a supreme expression of the Good.
An emphasis on idealizing the common person: After nature, idealization of the common "man" is the trait most associated with the Romantic poets. Impressed in various way by the ideals and fervor of the French Revolution, these poets wanted to celebrate ordinary people and show them in the best possible light in order to promote ideals of universal brotherhood. It can't be emphasized enough that, despite a few lone antecedents like Thomas Gray, this is a new thing. Poets before the Romantics used "shepherds" as color or to evoke the Classical age, but these shepherds were never real people. The lower classes were most often depicted as clowns, if depicted as all. The Romantics, in contrast, portrayed the lower classes as filled with dignity and grace, whose simple lives could be a model that the upper classes could learn from.
An emphasis on simplicity: Our language has changed, so sometimes Romantic poems don't seem simple to understand, but the Romantics put great emphasis (on the whole) on using simple, accessible language that everyone could easily understand.
An emphasis on lyricism: Lyricism is the expression of emotion. Over and over again, Romantics try to capture emotions in verse.
An emphasis on the supernatural: The Romantics moved away from the rationalism of Neoclassic poetry to examine the whimsical, including fairies, folktales, and magic.
An emphasis on memory/the past: The Romantics had a special interest in the color and magic of the Middle Ages, but they also emphasized the solace that a store of happy personal memories could bring.