There are a number of ways to interpret this poem. Looked at literally, as mizzwillie does in her post above, the poem resonates with ideas of loneliness, isolation and eventually a yearning for life.
However, if we read the poem more figuratively (metaphorically), we might see "The Lady of Shalott" as a poem about death.
The Lady in the poem is repeatedly associated with death, from Part I all the way to Part IV.
The first connection to death comes early in the poem.
The reaper, reaping late and early,
Hears her ever chanting cheerly,
Like an angel, singing clearly
Reaping is a common metaphor
for death as the act of reaping takes place as part of the harvesting of crops (e.g., when a plant has passed through its life cycle). The additional suggestion that the Lady's song and voice reminds the reapers of angels further links her to the idea of death (as angels are often figures of the afterlife).
The "web" that the Lady is said to be weaving might be taken to be a shroud (used to wrap a corpse). The Lady's tendency to reflect and look back (or backwards through a mirror) also aligns nicely with the notion that she is emblematic of death, seeing life only in retrospect.
But in her web she still delights
To weave the mirror's magic sights,
For often thro' the silent nights
A funeral, with plumes and lights
And music, came from Camelot
We see again here that the Lady is connected to the idea of death and the poem suggests that she enjoys hearing the and seeing the signs of a funeral at Camelot.
The Lady dies at the end of the poem and completes her journey, arriving in Camelot as a corpse. Before she dies, however, those passing nearby "heard her chanting her deathsong." The Lady of Shalott seems to be a figure representing death then, not only in her final appearance and in her relation to reaping but also as a reminder to any who pass by.
One might go so far as to compare the Lady of Shalott to a cemetery or burial ground. She is a symbol of inevitable death and severance from life.
This is not a cheery view of the poem, but this is not a very cheery poem. The "mournful" song that the Lady sings might serve as a nice analogy
for the poem itself. Yet there is a sense that the Lady is revered, even if she is isolated.
She is dressed for death and perhaps is even already in a casket in Part I of the poem.
A pearl garland winds her head:
She leaneth on a velvet bed,
Full royally apparelled
However, in her appearance and in her effect on those who hear her, the Lady does not bring melancholy. She is cursed to do her work of weaving, but she does not mind. She is alone, but she is unconcerned with the "churls" and "market girls" of Camelot. And when her work in front of the mirror is done, she departs in a final death.