The word that can sum up many of the themes in the book is "position". The word encompasses themes like class, wealth, social standing, and others. Gatsby's whole life is spent trying to attain money and status so that he can reach a certain position in life. That is what motivated him to move to West Egg, make money by any means necessary, and strive to win Daisy back. There is a position in life that he yearns for and will do all that it takes to achieve it.
Daisy and Tom on the other hand show how people can use their position to look down on others and live their life carelessly. As Nick says about Daisy, "in a moment she looked at me with an absolute smirk on her lovely face as if she had asserted her membership in a rather distinguished secret society to which she and Tom belonged". It is this superior mindset that allows Tom to cheat on his wife and allows he and Daisy to run away from the death of Myrtle. They need not worry about such things because they are too good for it. Nick sees it as a kind of carelessness. "They were careless people, Tom and Daisy- they smashed up things and creatures and then retreated back into their money or their vast carelessness...". They can use their wealth and position to escape whatever they choose.
The word "careless" also sums up one of the most important ideas in the book. Nick refers to Jordan, Tom, and Daisy as careless in one form or another. Their actions are careless and they are careless people. This is due to the ease of their life. These people live the decadent life of the roaring twenties that many of the writers of this era were criticizing. The mindless, indulgent, irresponsible life style where consequences are just an afterthought. Fitzgerald uses these characters to expose this life with their selfish actions. This carelessness can be seen when Tom and Daisy run away after Myrtle is killed or when Jordan is driving Nick through the city. These people do not worry about paying for their actions so they do as they please. Tom is not worried about hurting Daisy so he flaunts his relationship with Myrtle, his mistress. Daisy, in turn, goes off with Gatsby without a thought to her marriage. Consequence is a unheard of concept to these people so they live their lives without thinking about it.