Drupal sites can look like anything, but some of the most 'Drupally' things about them are either commonly overlooked or are a pain to change. Trying to implement designs that are counterintuitive to Drupal's patterns can be frustrating and very expensive, especially when there are ways to work with Drupal without sacrificing design control.
When designs neglect or disregard certain bits and pieces that are common on most sites, they become missed opportunities and can create visual inconsistencies in the overall look. With diligence and planning, designers and themers can design with these elements in mind, and in turn deliver a robust, polished result.
Conversely, some things are really difficult to accomplish with a Drupal site. Though not always impossible, some areas of customization generally do not merit the extra time and money required. When crucial to the site and designed with understanding of Drupal limitations and markup, specific difficult site elements can make a significant impact on user experience, but often need more than CSS.
The goal is to have a design that is created with Drupal in mind. It takes advantage of things that Drupal does well, makes it easier to theme, and implements the desired look with all of the Drupalisms considered. Knowing what to expect from Drupal allows you to make smart choices and maintain your visual intent.
Identify simple elements, that when 'undesigned', make a site look 'Drupally'
Discuss difficult but often impactful elements of Drupal sites that require extra finesse
Offer ideas about when designing with Drupal or against Drupal is appropriate