A marketing message is only effective as its ability to engage and compel its recipients. It is common for a message to become confusing or forgotten unless it is written clearly with direction and purpose. Several mistakes make content ineffective. 

1. Poor Grammar
   Great grammar is expected whereas poor grammar gets noticed. Copy writers are expected to compose content that is expressed with proper verb conjugation, voice, tense, pronoun use, etc. Readers appreciate well written grammar because they are able to understand and digest the material without being distracted by mistakes and mishaps.
2. Poor Spelling
   Many people are quickly distracted and turned off of a message by poor spelling. It is important to use spell checkers and review copy before it is published. It is key to make sure the was’s and were’s  and the there, they’re, and their type words are used appropriately.
3. Poor Formatting
   A run on sentence may be confusing and emotion killing. At the same time, a list of 50 bullets is intimidating. Content should be written with headers, paragraphs, and appropriate fonts and sizes. Blinking or scrolling text is rarely appropriate. The format of a document should help the reader find and read what they are looking for quickly.
4. Lacks Competence
   Copy should appear credible and indicate that some thought went into it. Writers should research some to approach an audience with valid and valuable information. All to often, blog writers and other editors allow content that seems useless. Readers will abandon it quickly. 
5. Lacks Relevance
   Copy should target an audience. It should be relevant to that audience. Examples, stories, links, etc. should all apply as valid, pertinent content. Content is made relevant when the composer writes for the audience with a purpose in mind. The copy should contain facts and concepts the audience expects.
6. Too Long
   Many points can be made in very few sentences or words. Long winded explanations become cumbersome and the audience becomes bored. The goal should be to get a point across in as few words a possible so the target may move on to the next step in the process.

From the Vauntium article: 6 Costly Mistakes in Content/Copy Creation