6. You Spoke Directly To The Reader
Unless you're Shane Black (and let's face it, very few of us are), it's not a good idea to break down established conventions so that you can directly communicate with the reader. On the surface, it might appear tempting to include little comments and messages to the person who could potentially get your screenplay made, as to win them round to your side, but you don't want to overstep your boundaries. Besides, it's never usually done well, and mostly succeeds in taking the reader out of the story.
5. You Keep Writing Your Character's Names In CAPS
When you first introduce a character, write their name in CAPITALS. From that point on, though, you really don't need to write their name in capital letters anymore (unless it's for a dialogue cue). It clogs up a screenplay and makes it surprisingly awkward to read when THE text LOOKS like THIS every FEW sentences. Argh!