Eliot's The Waste Land can be viewed as an exploration of the mentality or collective mind of the early twentieth century, a time in which a vast upheaval occurred in the way intellectuals viewed history, religion, and aesthetics. There are numerous ways in which we can relate Eliot's thinking to Freudian theory, but I would choose the levels of "awareness" Freud defined—the conscious, preconscious, and unconscious—as applicable to the approach throughout this seminal modernist work.
Eliot's writings in general are highly intellectualized. I use this term rather than simply "intellectual" because in my view there is a self-conscious, or even deliberately artificial, approach that typifies his poetry. The Waste Land is a depiction of the modern world overlaid with quotations from literature in different languages and cultural allusions that people like Eliot himself would understand, but often on a preconscious or even unconscious level. The opening of the poem:
April is the cruelest month, breeding
Lilacs out of the dead land. . . .
Those of us who have studied The Waste Land are immediately aware that Eliot is alluding to Chaucer and doing a kind of inversion of the underlying meaning, the positivity of spring, expressed in the opening of The Canterbury Tales. He is transforming it into a statement of pessimism. But on first glance, it might take even a well-read person a while to retrieve this from the "preconscious." Similarly, Eliot's vision of the modern world as a city of the dead:
A crowd flowed over London Bridge, so many,
I had not thought death had undone so many
is a paraphrase of lines from Dante describing hell. Eliot seems to have been obsessed with Dante and quotes him everywhere, but those who have not read The Divine Comedy would be at a loss in recognizing the source, and the total meaning, of this paraphrase and would not necessarily know it is a quotation without having studied The Waste Land in a scholarly edition.
But there is nevertheless an assumption—and I believe it is a valid one—that Eliot makes about his readers, which implied in the subtext of his whole poetic technique. The Waste Land operates most completely on the level of the unconscious, I think. This means that even if we do not recognize the literary allusions or are informed by others (either our teachers in class or the footnotes of a scholarly edition) of their meaning, we have a kind of primal sense of their significance. Eliot assumes his readers are in tune with the modern Zeitgeist. The fragmented, shattered-glass quality of The Waste Land is emblematic of a culture that is already post-historical in some sense, as if in the modern age, our thinking and conception of art consists of bits and pieces of the past, like the wreckage of a ship, that we are holding on to. This is our unconscious, our collective "past," like a troubled childhood that took place before our memory process began but which is nevertheless buried deeply within us and affects our every thought. For example, Eliot quotes from Wagner's Tristan and Isolde:
Frisch weht der Wind
Der Heimat zu,
Mein Irisch Kind,
Wo weilest du?
Eliot sees Wagner, for good or ill, as an indissoluble part of the European cultural heritage, this "unhappy childhood" submerged in our memory even if the specifics of it are not known (and the German quotation would probably not be known or understood by 99 out of 100 English-speaking people). It is like a bad dream now being forced into our conscious life. These preconscious and unconscious things thread their way through the poem in the midst of those elements of which we are, by contrast, immediately aware and therefore comprise our conscious life. Or, depending on our pre-existing knowledge, elements of the poem can hold multiple meanings, on two or perhaps even all three levels of consciousness. In any event, the complexity of thought in The Waste Land lends itself to multiple interpretations in connection with Freud, not merely the one offered here.