Great advice in this thread. I suggest that, while writing, you assume that the player/reader understands terms like "d6", "class", and "hit points". Don't waste valuable time defining common RPG components. Just get your rules down into playtest form.
In fact, keep "playtest form" in mind as your standard of quality. You're going to get a huge amount of feedback, so whatever you're writing will most likely have to be re-written. Don't labor on eloquent language; just pound out the rules in their most basic form. Your goal right now is to convey the bare mechanics, so you can test the SYSTEM. Worry about structure and presentation later. Much later.
If you spend too much time crafting paragraphs now you're just set yourself up for frustration later.
-z