Defining the "rich" West Indian family and the "most respectable" Girls Boarding Academy as "company worth having" is a direct commentary on the socioeconomic break downs of society and Austen's views / judgment on what makes a society or company worth having. In Emma, Austen uses Harriet and Mrs. Elton to have even more pointed conversations about who and what is respectable company.

Diana's concern about the sea air and perils of a visit to Sanditon sounds similar to Mr. Woodhouse's complaints about weather and travels impacting his health in Emma.

Letters are a recurring plot device in Austen's novels, from Knightley's declaration of love via letter to Elizabeth in Pride and Prejudice to Frank Churchill's consistent apologies in letters to his father and step-mother (Mr. and Mrs. Weston) in Emma. Here, they serve as an opportunity for Austen to introduce the reader to a few new characters who will factor into the story later on.

Compared to most of Austen's other works, this incident is an extreme amount of contact and interaction between characters from different "socioeconomic classes." The servants and coachmen etc. are often mentioned in passing. Emma is said to do some charity work but it is also mentioned in broad terms and the reader is never shown a scene of such acts.

Conversations about the importance or necessity of an occupation are rare in Austen's novels. If they occur at all they are about men and often in the context of them needing a way to provide for their future family. Within that context, this comment from Mr. Parker directed at Sidney and pointedly addressing his socioeconomic status seems out of character for Austen and is one aspect that differentiates Sanditon from her other works.

This is my favorite word because I like the way it sounds out loud