I think a lot of it has to do with the actual write-up of web content (html, css, javascript). A lot of people aren’t savvy with how websites actually get coded and much of that is to blame with how unintuitive, messy, and intimidating write-up can be. Thus, people resort to doing very simplistic designs, using templates, or (the worst) stealing code from someone else’s website and recycling the same designs over and over (web 2.0 styles anyone?).
It’s not like in print where you have visual tools like photoshop, illustrator and indesign to create you product. What you see there is what you get. With web, your design has to be translated into a complex organizational system where you find out that such-and-such an effect can’t happen without mountains of javascript, or that navigation just can’t work, or those images don’t load fast enough. That’s when, again, you just resort to tried and true methods and the web gets boring.
Jason Santa Maria is a rare case that he’s such a wonderful developer and designer. Thus, his back-end can support all the innovative features he wants on his front end (that wording came off mildly sensual). Most other people are either designers with little web development experience or developers with little design sense.
HTML5 and CSS3 are heading in a promising direction. With the help of wonderful people like Jack Zerby, write-up is starting to make a lot more sense semantically and complex processes that used to require round-a-bout solutions (like button styling) are much more simplified.