Themeforest is more strict now than before, Its because they want to reduce the review time. So they prefer to hard reject items rather than soft reject.
If your design, and the purpose of theme is valid and acceptable, other issues they can ask you to fix them. Rather than rejecting hard your theme.
Your Theme’s message is not clear what is this theme for , the slider is too simple and too dark, Laptop Gets all attention and the message written in left is not clear.
The design you are using for body, is available in many other themes its not unique, The color scheme need some work, the typography use Google fonts, And your theme should clearly establish the purpose what is it for and who can use it.
No one can provide you complete guide, but you should search as much as you can recent themeForest PSD templates, and WordPress themes to understand the difference in design quality.