I recently released a plugin called system information.
I wrote that plugin because in the last few months I have been struggling in providing support to some of the users of my other plugins. Some of them are not savvy enough to be able to provide me detailed information on their configuration. So, with the plugin, they just have to copy/paste in an email what's in the text box and I know everything I need to know to be able to figure out the problem they are experiencing with my plugins.
As that plugin has been invaluable to me, I would like to help other plugin authors by including the information that they need to support their users.
What kind of information would you like to see there?