Fitzgerald has been criticized for not fleshing out the love affair between Gatsby and Daisy sufficiently. We know that Gatsby is obsessed with Daisy, but we don't get much of a glimpse of what draws Daisy to Gatsby or about the relationship the two share. It would be helpful if Nick, for instance, could overhear a private conversation between the two of them that expressed their love—or if Daisy could talk to him about her attraction to Gatsby. Too much is left to guesswork. Gatsby's dream tends to swamp the reality of the affair.
A second issue might be the amount of time and space devoted to Tom Buchanan. Whenever Tom is around, he kills the joy. That is simply an aspect of his personality, but in a short novel we are subjected to too much of it.
From the first critics, such as Mencken, people have complained that the social justice aspects of the book are insufficiently fleshed out. Fitzgerald does deal with them, such as through the Wilsons and the Valley of Ashes, through contemporary songs he mentions, and through the Buchanans' butler, but some have said the book has too much focus on the rich.
Attached is a review that is a scathing critique of the novel.