I was recently reading this article over at The Economist. The content of the article aside, it made one other thing come to mind: with great power comes great responsibility.
As I began reading the article, and thus scrolling, all of a sudden my screen began to look a cluttered mess. A full-screen-width bar dropped down from the top with please-for-the-love-of-God-share-this-article-on-all-your-social-networking-sites buttons and a search box. Besides being redundant since those features are all already embedded on the page, it was distracting. About the same time a square box slides up from the bottom telling me I need to subscribe to the magazine, again covering up the content of the article. Again, there is already an advertisement-like area near the top of the page offering four free issues and telling you to subscribe.
The passive versions of these features I am fine with, but the two slide-in boxes are too much because they happen as one has already begun to read, thus distracting your attention and covering up the content you're there to see in the first place. It is a very in-your-face type of pressure that most people do not approve, just like extremely loud commercials.
Okay, so if The Economist is the Comcast of internet news, what's an example of dynamic pages done right in that area? I think the New York Times does it right. Go read an article (this one I chose at random) or simply scroll through it. Nothing pops up to annoy you, everything happens on page load. The one exception is when you reach the bottom of the article a box slides in--and not over the content you're reading!--letting you know of related articles you may be interested in. This is actually helpful rather than self-serving like The Economists's dynamic content.
So, while we are all enamored with the eye candy of modern Ajax development remember to take a critical eye to it and note those who are using it well. I think I'll go read some more NYT.