If we're talking web design, the answer is everything. Don't believe me? Ask Kathy Sierra. She'll tell you there's one way to be successful on the web—create passionate users. And the only way to do that is to be passionate yourself.
Writing valid XHTML markup is not really as difficult as it sounds. If you learn a few new rules, you'll be marking up code in valid XHTML in no time. The best way to familiarize yourself with valid markup is to view source on site pages that have passed one of the validation services, such as W3C's Markup Validation Service. Here is a quick rundown of the basics to help you get started.
Now that I've worked on a few projects with Drupal v4.7, I have to say that I'm impressed as all get-out with it. Personally, I can't think of any reason not to use it on a majority of projects. But Drupal is my personal preference, and I'm not sure it would be the best one for everyone.
For the past few weeks I've been stuck in bed because of a minor illness. For two weeks my primary contact with the outside world has been through this laptop, and it has been an eye-opening experience. Never before did I grasp the true power of the Internet. Oh sure, I knew all it was capable of, but knowing and being completely dependent it on are two different things.