Here is something that really gets my goat. Websites with bad Visual Hierarchy. Here is a really great post that goes through the basics.
It's a great read, so I suggest you give it the once over! At it's core, however, the article talks about how as designers it's our job to use colour, texture and shape to portray the function of a website's interface. People are visual thinkers at heart, and process visual data much quicker than reading text. As designers, it's our job to ensure that the design decisions we use make sense and do not serve as a barrier to to whatever it is a user came to your website to do!
So what the hell am I talking about when I mention Colour Context? The foundation of good colour context suggests that we use repeatable design decisions throughout our website in a way that makes sense.