Empty States
Empty states provide us with the opportunity to improve the user's experience by helping to avoid confusion and keep them engaged. A useful empty state should provide the user with information about what is happening, why it's happening and what they should do about it.
Hierarchy
A good visual hierarchy can help guide the user through the entire user experience of your product. You can achieve this using things like size, color, contrast and whitespace. The lack of visual hierarchy will result in your end user being unable to focus their attention on what's important, thus making your application unusable.
Loading States
Loading states are very important because they let a user know that nothing is wrong and that something is happening. Imagine uploading an image/file or accessing a very data heavy page with no visual feedback that anything is happening. That visual feedback will keep users from becoming confused and may even keep them from leaving.
Organization
It is important to organize your UI's content, components and features so that your users don't get overwhelmed or lost while using it. Instead of providing users with every tool and/or feature up front, you'll want to present users with what they need only when they need it. A good way to do this is to divide your tools and features into categories and then choose where and when to display each category.
Whitespace
Whitespace is generally brought up when discussing Hierarchy, but since it is one of the most important aspects of UI/UX design, it deserves its own section. When used properly, it can improve the readability, legibility and user comprehension of your UI. If you did not implement any UI/UX best practices except for improving the use of whitespace, you would still vastly improve the usability of the interface.