A big addition, like isocryd said, would be to add a preview generator with the seed. i.e. when the seed is generated it loads a 3D preview of what the seed will look like.
You can do this in flash, find a way to preview it in a 3D preview
or
You can make a site with a java applet that loads the minecraft seed into a quick preview and have this file link to it via the _GET feature of the browser's address bar (the ? question mark at the end)
I think that would be a great addition to this, and I would definitely use it more :D
but for now I give you a 4/5