For me, clarity is the most important. Poor grammar is okay if I understand what you're saying. Learning disabilities, ESL, etc. are forgiven.

Things that I'm less patient about:
* Not using paragraphs. If you're writing more than 4-5 sentences in electronic communication, you should probably start a new paragraph. My brain refuses to process large blocks of text
* Unstructured, cluttered writing
* Verbosity without purpose
* Chat-speak

Clear communication is a sign of courtesy. You can try, to the best of your ability, to show your reader that you have made an effort to present yourself clearly.