This is going to be a serious problem for the adoption of web apps. Current problems with web apps are things like lack of keyboard shortcuts (without the browser getting in the way). Browser chrome wasting space in the UI and an overall more "laggy" feeling than a native app.

All of these problems however can (have?) be improved upon. UI consistency will be the last great bastion however, whenever I use a web app I have to spend a considerable amount of time figuring out where everything will be in the UI, this is even true for things like gmail.

Perhaps there will become a defaco standard widget set for web apps?