One of the greatest things about the Drupal CMS is that it can support *any* look and feel you want it to in a just click of a button. Or at least it can once you have the theme you want designed and "Drupalized".
Themes/designs are a very unique part of a web site, because they they simulataneously are "just" a wrapper for the content on your site - while at the same time being *the* main element which keeps your site from just being a bunch of text splattered on a page (screenshot of this page without an active theme).
So, the visual design of your site is one thing in itself, but it's only a beginning, and in a proper site construction a theme should start out literally as a 'pretty picture', mocked up in Photoshop for easy experimentation/changes.
What happens after this stage in theme development is an entirely different case altogether, because now we're moving from a 'pretty picture' to working code, which will affect browser compatibly (do Internet Explorer 7 users see your page as a blob?), download speed, and usability (does the link to the contact page not work for Firefox users?).
The coding stage is where expertise and testing become necessary in order to keep a your design from becoming a nightmare in terms of usability, compatibility, and flexibility.