I don't see a forum for ArcGIS.com yet, so I'm using the forum for ArcGIS Online instead.
The default setting of the new site ArcGIS.com is not "Show: All Content" but rather "Show: Web Content Only". This prevented me from seeing, at least initially, a layer that I knew was there (having found it previously in ArcGIS Online), as well as others that could be used with ArcGIS Desktop. It took too much time to figure out why, and many users might just miss the other content. So I think it's a serious mistake to make "Web Content Only" the default rather than "All Content".
I'd suggest a set of checkbox filters that appear in the drop-down. Then people could choose different combinations as they see fit. For example:
[ ] Maps
[ ] Web-viewable
[ ] Downloadable:
[ ] ArcGIS Desktop 9.3
[ ] ArcGIS Desktop 9.3.1
[ ] Apps
[ ] Tools
[ ] User-Supplied Content
As usual, clicking on "Downloadable" would select everything contained below it.
The default might be all of these checked, but changes would be saved in a cookie for later access.