We love making designing websites without worrying whether they can be coded or not. We love to shape websites rather than be shaped by the technology they are built with. We love flexibility. We love choice. ExpressionEngine is very good at all of this.
A surprising number of content management systems impinge upon design... overly so in our opinion. We like to design knowing that the CMS can handle whatever we choose to throw at it. ExpressionEngine is particularly good at letting us create as many layouts and page types as we like - with ease.
When you choose a CMS you want to make sure you can't just edit content, but that it can use that content intelligently. For example, a page may need to display different content depending on what the visitor has chosen. A powerful engine like ExpressionEngine can achieve such things. Entry-level CMS like WordPress can't.
When it comes down to it, WordPress is essentially a Blog CMS that is powerful enough to make simple websites. It is very good at doing what it does, but ultimately it is limited in scope. As soon as you want to do more with it, it has the tendency to place insurmountable hurdles in the way. There is nothing worse that developing a site only to find some way down the line that the technology it uses is not up to the job when it comes to expanding it.
ExpressionEngine on the other hand is a proper powerful, flexible CMS - a million miles more future-proof.