Quote 4
"That is an expression, Sir John," said Marianne warmly, "which I particularly dislike. I abhor every common-place phrase by which wit is intended; and 'setting one's cap at a man,' or 'making a conquest,' are the most odious of all. Their tendency is gross and illiberal; and if their construction could ever be deemed clever, time has long ago destroyed all its ingenuity." (9.16)
Marianne expresses her dislike for perfectly normal colloquial speech, which she finds too crude entirely. While Sir John could be a bit less direct and blunt, we have to say that he does manage to communicate more clearly than some of the other characters. Perhaps some of Marianne's troubles come from her desire to phrase everything poetically.
Quote 5
At last he turned round again, and regarded them both; she started up, and pronouncing his name in a tone of affection, held out her hand to him. He approached, and addressing himself rather to Elinor than Marianne, as if wishing to avoid her eye, and determined not to observe her attitude, inquired in a hurried manner after Mrs. Dashwood, and asked how long they had been in town. Elinor was robbed of all presence of mind by such an address, and was unable to say a word. But the feelings of her sister were instantly expressed. Her face was crimsoned over, and she exclaimed in a voice of the greatest emotion, "Good God! Willoughby, what is the meaning of this? Have you not received my letters? Will you not shake hands with me?" (28.6)
Again, we see a kind of odd disconnect between modes of conversation, even when two people are standing face to face. Willoughby is playing the stiff upper lip society guy, while Marianne, unable to pretend, speaks directly – a rare occurrence in any of the conversations we've seen.
Quote 6
The sort of desperate calmness with which this was said, lasted no longer than while she spoke, and was immediately followed by a return of the same excessive affliction. It was some minutes before she could go on with her letter, and the frequent bursts of grief which still obliged her, at intervals, to withhold her pen, were proofs enough of her feeling how more than probable it was that she was writing for the last time to Willoughby. (29.2)
The only clear line of communication available here is letter writing – it's only in written form that Marianne can say what she was unable to say to Willoughby in person.