I originally wrote this piece a few years back, but I believe that the principles still apply nowadays.
During my career I worked in different companies that are dedicated to the design and development of websites. On several occasions I was requested to add different features to commercial websites that even though I knew that they would be counterproductive, I had to add them anyway, as that was my job. Once I started freelancing, the truth is that things didn't change, only now the requests were being made directly to me by my clients, and even though I pointed out some of my concerns to clients with whom I had a good relationship, more often than not I had to do it anyway, after all it is the clients decision.
However, if you are reading this article, it's because you already have an interest to learn about common errors made in commercial websites. If you intend to turn your website into a good marketing tool, then here are some things that you should be aware of while having your website design.