I think you must seek it more into the 'behind-the-scene' of the webdesign (technical stuff, that a normal visitor doesn't see at first sight) then into the design itself.
Allot of design businesses design website's, but often those are more eyecandy then well built website's. So in order to make you stand out you must offer the client more 'add-ons' for their website, and even more important: the support when something is wrong.
I work at a design business myself, and we really lack support to give to our client after the website's is built. Therefor we can only built a small to normal sized website for a client because we just don't know any better (and after all, it isn't our specialty ... it's 'design'). We are now starting to built with Joomla, but even that at some point will be too much for us to handle.