Ill conceived, poorly written
This was the judgment of a reader at Knopf of a manuscript called “The Bell Jar,” submitted by one Sylvia Plath.
She's not the only writer of note to have received damning rejections from publishers early on. Sometimes early work might be bad, but more often it seems a case of painful subjectivity of judgment, or of publishers failing to recognise genius when it comes across their desk. It also draws attention to the difference between what's sellable and what's (eventually) great. The spirit and the machinery of literature are often at odds.
Here's a bunch more rejection letters at The Atlantic, including Vladimir Nabokov, Gertrude Stein, and Jack Kerouac.