Most of what is known about web design was uncovered years ago, and now it is just technology that changes with new bells and whistles. To that end, it really is a wise move on your part to discover a thing or two about web design fails (mistakes) and how to prevent them from happening on your sites.
One of the first things people want to know when they land on a site is what is the purpose of it. If you have been online much, then you have landed on sites that just seemed a bit scattered. You just cannot induce any discomfort on your visitors, with your site, and have any reasonable expectation for them to continue using it. The way you present the copy, or text, of the site is super important; it must not be a chore to digest. Big black blocks of text will intimidate people and does not facilitate skimming and scanning at all. Think about using more white space because that just makes it look easier to read (scan) and people will do it. Avoid trying to get to artsy with the fonts used for your text because that is extremely hard on the eyes. We think many people understand this, it is just that they do not yet understand what to do or how to approach it.