I am wondering and looking for some general reasons why there is so few full AJAX websites. Good example would be Gmail, which really uses a lot of the modern web technologies. But there is so few of these and almost all the websites use classic "request -> pageload" approach.

I am going to be developing website with most requests completed by AJAX call, as there is need for as few page loads as possible. So I am looking for some general advices or opinions about this.

My personal big concern is search engines, because if most of the content is at one URL, how do you make that AJAX content crawlable ?

There are probably more these questions which rise up, and I am looking for these to prepare for these problems before I start developing and it's too late to fix them.