My top 3 most important things about your web design are not going to be the same as another web designer's, so keep that in mind as I share what I believe are the top three.
Navigation
What is it the most important? Because it drives me mad every time I visit a site and I cannot easily find where I want to go next.
Menus that try to get too fancy, have too many options, or are in non-standard places are going to frustrate website visitors.
Simple and straightforward are still the best choices when it comes to navigation.
What about mega menus? They have their place, especially on e-commerce sites, but they should be used sparingly for non e-commerce sites, unless it makes sense for your specific website.
Animations (when there are too many)
Animations are popular, but there is such a thing as too much. Too many animations can cause the brain to lose focus. Instead of drawing the eyes to where you want your visitor to look, one animation after another, especially when they flash and move rapidly, can have the opposite effect.
Animations (paralax, reveals, flying images, text movement, etc.) should be used sparingly and with purpose.
User-Friendly Experience (Three-Way Tie)
This one, at least for me, ties with calls-to-action and white space. All three are important, and if you give your website visitors a user-friendly experience, they are more likely to click on a call-to-action. If you use more white space, they are more like to see your call-to-action.
Don't make your website users work hard to find content (goes hand-in-hand with navigation), and when they find the content they want, make it as easy as possible for them to move from one page to the next or one element to another. Don't distract them with so many graphics, animations, and colorful backgrounds that they don't know what they are looking at.
White space helps the eyes focus on the website's important content and helps calls-to-action stand out more, all of which creates a better user experience.
While time spent on your website is important for analytics, I believe it is better to have someone visit your site, find what they want, and click on your call-to-action. If it takes only a few minutes for you to gain a client who contacts you or a shopper who buys, then that is a win.
Conclusion
Typography, layout, colors, and more are all part of the big web-design picture, but if you get these three elements wrong, I guarantee you'll lose more website users than you gain.