Clarity of thought. Organize what you want to say in some kind of order. Avoid shortcuts and regional idioms. Try to read it as someone else will. Spelling and punctuation are very important, one little mistake can change the whole meaning of what you are trying to say. For instance, if you want to ask..but is it legal? and you actually say..yes, but it is legal.. switching those two little words changes the whole thing.
Getting really picky, many people have trouble with the difference among, to, two and too. They are not interchangable, and mixing them up causes confusion to the reader.
Lastly, do not ramble.