Keep in mind that you are writing for medical students, physician faculty members and non-physician faculty members including those in the humanities, natural sciences, social sciences, public health.
A bad essay might be poorly written with multiple grammatical and spelling errors, or (much harder to pick up with spell-check) use of fancy vocabulary builder words that just don't fit or flow. Another spell-check problem is homonyms (there/their hear/here, even cord/chord), The content of a bad essay may be so cliched that the reader gets a feeling of deja vu -- have I already read this essay three times this season? Common themes... inspired by grandparent's death (or grandpa wouldn't have died if he had known to take better care of himself), I admire my pediatrician (or ortho surgeon) and want to be like him, Mountains Beyond Mountains inspires me to be the next Paul Farmer, as an infant I grabbed my daddy's stethoscope and never let go. A special kind of cliche is the "screenplay" style essay that usually starts with lights and sirens and then becomes a dialogue between the applicant and a doctor or sick person, or an interior monologue.
Now some essays based on one of these cliches can be just average while some go over the edge to truly bad.
An excellent essay is usually well structured, well told and draws one in. While not pre-med in any way, I thought that the style of writing in The Glass Castle reminded me of an excellent PS. (Also a good book in terms of the triumph of the human spirit over the tragedy of alcoholism and mental illness and the strength of familial love and forgiveness.)