Being diverse in your designs can really help to grow your comfort level and accommodate more clients. If you are used to building simple small fixed width web sites you could really be limiting yourself. Experiment with fluid layouts some or maybe even mobile design and you could find yourself with many more opportunities. Improve your Photoshop abilities and you could provide that next great look that someone is looking for instead of the plain old boring graphics every website has.
More often than not clients today want more than just the simple website that so many have been asking for and getting over the years. They expect more out of their web professional and you should be looking to improve yourself in multiple areas. Once you stop pushing yourself to learn new things, you will soon find yourself left in the dust by developing technologies and techniques.
To be diverse in web development you have to constantly be learning new coding techniques. You can choose to specialize in a certain language or be very good at developing in a certain CMS but you will always run into an opportunity where a client may need something different from your specialty. You may be good at WordPress, but you have a client already using Drupal that needs some work done. Perhaps you are switching them from Drupal to WordPress. It would be helpful to know the basics of Drupal to recover the most that you can from their current website.