Poor grammar and spelling.
Spelling poorly (at least in the writings I've noticed recently) is usually the result of lazily depending on spellcheck. Spellcheck corrects the wrong word for you, so words like inconvenience become incontinence! Other frequent occurences include confusion between:
- ... and so on...
The crimes against grammar usually include:
- run on sentences
- apostrophe plurals (argghhhh!!!)
- vague pronoun references
- misplaced modifiers
- verb conjugations (especially irregular past participals and perfect tenses)
- semi-colons and colons used incorrectly (argghhhh!!!)
When I first started creative writing, I had been teaching grammar and composition for so long, I felt confident in my ability to at least write clearly and correctly. In some ways, that was half the battle right there.
I realize that online communications are often informal, and are often done on the fly. Plus, I must confess I've had my share of typos and grammar goofs, but things are rarely perfect.
However, if you want to get published, and maybe someday quit your day job to be a full time writer, it's a good idea to develop good habits and use them 99% of the time.
I think the point is that you must know what the rules are before you decide when and where to break them.