Having looked at the Compare template on ArcGIS Online a few enhancements came to mind...
- An option to select different maps for the different views
- A search location / address facility is, I think essential
- The ability to turn off layers (seems to be called content elsewhere)
- Print capability, measure tools etc
- Also the title bar is very deep, which isn’t good when the purpose of a map is the map so make this shallower. It would also provide room for other tools / options.
These would make something that has potential but limited use be far more dynamic and usable for different sectors. There's an example that shows it's not impossible at
http://www.esri.com/software/landsat-imagery/viewer :-)