Since you’re still working on the public Marketplace, this is more a suggestion, rather than feedback.
I just thought it would be cool, if you could play test some of the things that will be offered on the marketplace right in the browser. Since you now have HTML5 support, this should be doable, right? Of course that doesn’t make sense for all types of content, like editor plugins or huge content examples. But for simpler assets, like fully textured, animated and scripted Blueprints it would be quite nice. You could allow the uploader to upload a simple map that can be used to try the asset.
Also, I hope there will be a standalone website that can be accessed with my web browser, like Steam for instance