I feel that poorly written secondaries include one, some, or all of the following:
-grammar or spelling errors
-poor communication skills (it's hard to decipher what you mean)
-responses that do not actually answer the question that was asked
-responses that make the applicant seem entitled, conceited, or otherwise unappealing
-racist, sexist, homophobic, or similarly offensive ideologies
-references to inappropriate topics
However, it is important to consider that the goal in writing your secondaries is not to be merely not poor, but to be good. Adcoms see tons of applicants with good stats, ECs, etc., so you want your secondary responses to be strong so that they can set you apart as an exceptional applicant.