In "Ode to a Nightingale," Keats (the speaker) wishes to be like the nightingale because it has no worry or no concept of mortality. Keats was always painfully aware of death. He feared dying at a young age (which he did). In the second stanza of this poem, he speaks of an escape from this awful awareness, asking for a "draught of vintage" so that he might lose consciousness and not have to deal with thoughts of his own mortality.
When he says he has "been half in love with easeful Death," he temporarily looks at death also as an escape from the reality of facing his own immortality. However, this escape would be an acknowledgment of mortality and in death he would no longer hear the immortal song of the nightingale.
He has only been "half" in love with death because it was a fleeting thought that death was an escape from thoughts of death. Being "half in love," he was just temporarily entertaining the notion that death would be a relief from thinking about death. It is a temporary or "half" thought because he immediately realizes this would be a surrender; not an escape.