This one was annoying me to no end. Under Windows XP, if you used the "X" button to close your Live Messenger window, it minimized to they System Tray instead of the task bar. For some reason, Microsoft changed this behavior with Windows 7. Instead of minimizing to the System Tray, the button stayed on your task bar.
Personally, I much prefer instant messenger apps, and others that are basically running in the background, to sit in my system tray, saving taskbar space for applications that are actively doing something.
Well, a quick web search turned up this lovely article over at How-To Geek. Long story short, you can get the old behavior back by changing the application compatibility mode. If you set it to Windows Vista, it will minimize to the tray instead of the taskbar. This certainly indicates it's something specific to the Windows 7 interface. I haven't played around with turning of Aero to see if that makes a difference, as I rather like the Aero look and feel.
While this hack works for now, it makes me wonder what Microsoft will do with the upcoming Metro interface.
This is the type of thing where the user should be given the choice. Some apps alread minimize to the system tray under Windows 7/Aero. Specifically, the Google Talk app does. Further, applications used to ask how you wanted them to handle minimizing, either to the tray or to the taskbar.
It should be about control, and customizing, and the ability to have a system that performs how you want it to.
Oh well. For now, at least, application compatibility settings handle the need without seeming to break anything.