...which would display the discussions in all of the groups you currently belong to, sorted by latest post. This would be easier and faster than going into each group one at a time to check for new replies.
This is something I want too. For now, I'm using this: http://www.glitch.com/groups/RHV10V06BH5240F/discuss/2338/
which shows the last post beside each group name, so I can tell which ones have had recent activity. It doesn't sort them, though.