What are the major themes in Lyrical Ballads?

The basic themes of Lyrical Ballads are nature and the imagination, the purity and natural wisdom of children,  an emphasis on the common man and common speech as opposed to poetic diction, and the notion that poetry can achieve the sublime by treating ordinary subjects. To quote another eNote (which you can find ):

...Wordsworth wanted to explore how one could attain profound truths and sublime emotional experiences via the imagination. In other words, this process is about understanding the extraordinary while experiencing the ordinary.

Examples are easy to find. In Wordsworth's "Tintern Abbey" the poet is inspired by the beauty of the ruined church:

To them I may have owed another gift,
Of aspect more sublime; that blessed mood,
In which the burthen of the mystery,
In which the heavy and the weary weight
Of all this unintelligible world
Is lighten'd:—that serene and blessed mood,
In which the affections gently lead us on,
Until, the breath of this corporeal frame,
And even the motion of our human blood
Almost suspended, we are laid asleep
In body, and become a living soul:
While with an eye made quiet by the power
Of harmony, and the deep power of joy,
We see into the life of things.

The poet's sensitivity to natural beauty, combined the transformative power of his imagination, brings upon that "blessed mood" in which he is about to reach the sublime, or "see into the life of things." 

Other poems in the collection are inspired by the troubles of common people: in "The Last of the Flock," the poet encouters a shepherd who has sold all his flock to feed his family; in "We Are Seven," the poet encounters a little girl who tells him of the death of two of her siblings.

Coleridge's contribution to the edition, "The Rime of the Ancient Mariner," Is much different in tone, but shares a central concern with nature. Unlike Wordsworth, Coleridge problematizes man's relationship to nature -- man is separated from nature by his pride. The mariner's killing of the albatross is a kind of crime against nature, and it is only through suffering that redemption can be found. (See  for more.)