I would be willing to bet that some people don't like installing a plugin and that is why the other photo sites don't do that.
This is basically the same thing. Except that it's right in the browser rather than a separate tool. And looking at their product, it seems to make a lot of sense.
But, I wouldn't be surprised if the thought process for many of the photo sites was: do we make one uploader for Windows and one for Mac or do we make a plugin for Safari for Mac, Firefox for Mac, Safari for Windows, Internet Explorer… (without even thinking about versions of OS)
* edit: apparently there are no limits but the one sites put themselves. The biggest limit is the need to go file by file.
No, there's no such limit. Although most people have their web server set to disallow more than say 10 uploads at once.
You can do some awesome uploaders with Javascript, and either iFrames or Flash. But to do the camera import you need to get outside the browser sandbox, so a plugin does seem like the best way.
I agree, it looks like a compelling product. Many of my family members use sites like EasyShare instead of a "better" site like flickr because they can import directly from the camera.