I love that users have the ability to configure their own popup windows when creating a map in ArcGIS Online without having to get into any code, and that there are a few options for them to start from (list, description, custom, none). However, I feel like this is only the beginning. When there are many attributes for a given layer, it would be great to be able to organize them across a tabbed window, instead of being forced to use a custom scrollable panel. This way, one tab could contain a free-form description, another tab could contain a list of selected attributes, another with an image (either from a feature attachment from from a URL from that attributes) and maybe even another with a list of related links. The 'custom' option will let you do most of this, but the scrollable panel is not as good for organizing a lot of data as a tabbed popup would be.
Something like this (http://www.masscec.com/index.cfm/cdid/12451) would be nice, and I'm sure you can do better than Google on this.