One of the things that I like about developing for the web is that there is a culture of openness about web development techniques. There has to be, all the source is out in the open already. In fact, there is significant pressure on web developers to write good code because it's all hanging in the breeze.
One of the impacts of this is rapid migration of new ideas. Part of this is down to the ease with which one can pop open firebug and have a poke around inside someone's html and css. It makes for easy remixing.
I suspect this is probably the hardest part for adoption of Flash and Flex - all the really cool apps are not open for review and learning. Actually, the same can easily be said of back end development. *cough* Google *cough*