The thing that bugs me the most about spectator mode is that when the game is over, there is no notification to the spectators. When I log on and see a friend who I want to play with in a game I usually spectate it thinking to join them when the game is over. Sometimes I end up watching until the end (when its difficult to predict who will win or a surrender vote happens) and my friend is already queueing again before I knew that their game was over.

It would be really neat to have maybe just a little popup that tells you when the game has concluded in a corner somewhere. It may or may not be possible depending on how spectator mode is implemented, but it would be a very useful addition if it was possible.