1. Lack of overall vision
This mistake will lead to an end product that has no main goal. The planning stage of web design is very important to you can understand why you are building the website, who you are targeting, and how you can target that audience.
2. Following trends
Following trends can be a hit or miss because your website will look outdated sooner. Also, just because a website style is considered "trendy" does not mean that it is a good design.
3. Not aligning to a grid
This mistake can leave your website looking choppy and unprofessional.
4. Bad typography
Don't use too many different fonts in one website. It is important to make your website consistent.
5. Poor photography
Bad photography can make a website lose it's beauty. I personally would add that generic stock photography with smiling strangers is not so great either. High quality is a must, but not so high that it would lengthen the loading time of your website.
6. Poor icons & logo
A badly design logo and poor icons are another mistake people do. I often see websites that have a decent design, but their logo is either a.) stretched out b.) not transparent/seamless c.) uses terrible colors/fonts. Your logo is even more important than your website, because it represents who you are/who your company is.
7. Leaving things unfinished or broken
This is very unprofessional. If something is unfinished it is best to not leave it live.