In a nut-shell, Joel Spolsky argues that Gmail and other AJAX-etc. tools of today are missing crucial support for things like cut and paste (e.g. how to get an image from Gmail to Flickr?), opening the door for a new browser-environment-add-on-thingie or software development kit to come along, allowing programmers to create more powerful webpages... which will then be in competition to Gmail etc. and one day overtake them because they offer more to the user.

http://www.joelonsoftware.com/items/2007/09/18.html?provocative_title

(Google does have an SDK though for DHTML apps – GWT, the Google Web Toolkit...)

Interesting. I think Google's trying to address this with the Google Mashup Editor. If they extend it to incorporate more Google services / web widgets it might be able to provide this sort of framework.

I think the best route will be for Google to implement more features into Google Toolbar to fix/add new features. For example, Toolbar for Firefox improves Google Docs' Undo/Redo function.