No cliches or unclear pronoun reference. The few errors do not distract the reader. Sometimes the reader cannot follow your paper because of the grammar used. Mostly, however, the errors are only a question of style. Some unclear pronoun reference, dangling modifiers or errors of tense that confuse the reader.
Errors make the reader stop, pause, backtrack, and re-read often.
Many run-on sentences or fragmentary ideas. Frequently unclear pronoun reference. Cliches or slang used inappropriately. Many typographical errors on every page. Poor grammar makes the work almost incomprehensible.